track the ballast representatively accounts for about two-thirdsof the total moment of lateral resistance. Loose sleepers andkinks in the alignment are of especial significance to the stabilityof welded track so that a high standard of maintenance isessential for this sort of permanent way.

25.7.1 Fastening welded rails

The need to fasten the rails in order to get a stress-free rail atabout the mean of the annual temperature range would mean, inmany parts of the world, a very short season for laying weldedrails without artificial assistance. This assistance is normallysupplied either by heating the rails by propane gas travellingheaters or by using hydraulic tensors, so that when fasteneddown after extension by heat or tensile pull, the rail has a lengthequivalent to a stress-free condition at a temperature betweenprescribed limits (21 to 270C for the UK). In this way it ispossible to lay welded track all the year round. The pull requiredto extend a 56 kg/m rail is of the order of 1.6 t/°C. The tensorshave a capacity of about 701.

Figure 25.6 Hydraulic rail stresser. This has a 70-t pull and380-mm stroke. For pushing action, the rail clamps are turnedthrough 180°. (Courtesy: Permaquip Co. Ltd)

Welded rails are not normally laid in Europe in curves with alesser radius than 600 m. Long welded rails are made up by flashbutt welding of standard rail lengths into welded rails of 200 to400 m length in depot and welded into continuous welded railsby Thermit welds, except in the USSR where a transportableflash butt welder is applied to join rails in the track.

It is normal practice in Britain and France to install slidingswitches at the ends of long welded rails but in Germany this isnot done, reliance being placed on very firm fastening of thewelded rails where they connect with jointed track.

The advantages of welded track include an extension of raillives by about a third, a reduction of on-track maintenance byabout half, a dramatic reduction of rail breakages, an increase inrunning speeds, less damage to the formation, increased sleeperlives, improved ride comfort, a reduction of traction energy ofup to 5%, and a reduction in train noise. In effect, welded rails,prestressed concrete sleepers and self-tensioning fastenings arecomplementary to each other in extending the life and reducingthe long-term overall cost of permanent way.

25.8 Switches and crossings

All railway switch and crossing work is built up of three basicunits: (1) switches; (2) common or acute (angle) crossings; and(3) obtuse (angle) or diamond crossings. Layouts of switchesand crossings are illustrated in Figures 25.7 to 25.10.

25.8.1 Switches

The heel of a switch is the point from which it is free to move.Early switch designs had a loose heel formed by a semi-tightfishplated joint but this type is now rarely used outside ofsidings. It is now the general practice for the heel of the switch tobe formed by a bolted connection through a block betweenswitchrail and stockrail so that the switch joint is well behind theheel. The length of the switch planing on the bead of the railvaries between about 1.6 and 11 m and most railway administra-tions have a range of two to eight standard switches to meet therequirements of short leads in depots and various speeds oftraffic on the running lines.

Originally, ordinary or straight planing of switch blades wasuniversally applied but during the last few decades curvedplaning, in which the switch rail is bent elastically whilst beingplaned on one side of the head, has become general. In this waythe curve running through the lead is continued to the switchtip. A recent practice is to apply a larger radius to the planing ofthe switch than is applied to the switch beyond the end of theplaning to give a more robust switch tip with a finite entry angle.

25.8.2 Crossings

Crossings are described by the angle at the crossing nose. Incontinental Europe it is common to state this in degrees but overthe English-speaking world the angle is usually described as 1 inN. However, the angle of a crossing expressed as 1 in W variesslightly according to the measuring convention adopted. Thus if1 in 8 is obtained by measuring 8 units along the centreline for 1unit of symmetrical spread normal to the centreline (centrelinemeasure), then measuring 8 units along one leg of the vee for aspread of 1 unit measured at right angles to that leg (right anglemeasure) will give 1 in 7.969, whilst if 8 units is measured alongeach leg to give a spread of 1 unit the crossing size will be 1 in8.016 (isosceles measure). It is therefore necessary for anyrailway to have a specific mode of measuring crossings which isknown to all supplying manufacturers of crossings, or to statethe angle in degrees.

In the UK fixed diamond crossings are limited by Ministryrequirement to be not flatter than 1 in 7.5. In continental Europe1 in 10 is the general limit. Where a line crossing flatter than theallowed limit is necessary, the points of the diamond crossingsare constructed as switches and have to be set to suit the trainmovement desired.

There is no official limit on the angle of common crossings but1 in 28 is about the flattest angle at which the crossing nose hassufficient robustness. Crossings are either made up from rails orare cast in 12 to 14% manganese steel. Built-up crossings havethe inherent limitations of bolted assemblies, and of recent yearsit has become general practice in the UK to weld the vee sectionand Huck-fasten the wings. In general, it is preferred practice tohave cast crossings for high-speed or heavily worked lines usingbuilt-up crossings for less exacting duty. It is also preferred touse cast crossings for angles flatter than about 1 in 20. In the USit is common practice for the nose section of the crossing to be amanganese casting to which ordinary rail section wings areattached to form what is known as a rail bound cast crossing.

Figure 25.10 Part-welded crossing in lead on prestressedconcrete bearers. (Courtesy: British Rail, London Midland Region)

25.8.3 LeadsThe design of railway connections is based on standard leads,i.e. on specific combinations of switches and crossings formingturnouts of specific length giving specific departure angles.Exceptionally, as at important junctions where space is limited,leads are specially designed on the basis of standard switcheswith special crossings, e.g. instead of utilizing either a 1 in 8 or 1in 9 crossing, a crossing is made to a fine decimal angle, such as 1in 8.319, but this is avoided as far as possible in, the interests ofstandardization. However, reference to a standard textbook onpermanent way, such as British Railway Track by C. L. Heeler isrecommended to civil engineers not familiar with fieldwork.

25.9 Slab track

Slab track installations have generally been constructed eitheras a continuous ribbon of reinforced concrete or as a series ofprecast units (usually specially designed sleepers) connected intoa reinforced slab by in situ concrete. Both systems have been putinto service in various parts of the world, .but in general the

Figure 25.7 Timbered layout oflong switches

Figure 25.8 Obtuse crossingFigure 25.9 Cast common crossing. (Courtesy: Edgar Allen & Co.Ltd)

continuously constructed slab (see Figure 25.11) has beenfavoured both because of its higher accuracy and its lower cost.The use of precast units has been restricted to sites with limitedpossession periods or to very short lengths.

The benefits of this type of track can be summarized asfollows:

(1) It provides an accurate and stable track geometry. In tightlocations it is not necessary to allow for the effects of trackmovement so that speed restrictions can be less onerous.Elsewhere, the high standards of accuracy that are attain-able allow for high-speed running without the problems ofmaintaining alignment and top that are associated withconventional track, thus providing a considerable economicsaving.

(2) The peak loading on the formation is reduced to somethingless than a quarter of that produced under conventionaltrack. Consequently, it is possible to use the system toreduce formation problems, in many cases providing a goodtrack geometry where most other track systems wouldrequire considerable and frequent effort to do so.

(3) The day-to-day maintenance cost of slab track is very low,requiring little more than standard patrolling. Additionally,however, experience has shown that periodic replacementcosts are also greatly reduced since rates of wear anddeterioration are much slower than for most other forms oftrack.

(4) The requirement of conventional track for considerable andrecurring engineering possessions to permit the working ofon-line maintenance equipment is eliminated.

(5) The hazard of track buckling, either vertically or laterally, iseliminated. Conventional sleepered track has a safety factoragainst lateral buckling which, at the high temperaturelimit, is typically of the order of 1.3. Some maintenanceprocedures can severely reduce this further. However, withslab track the full weight and stiffness of the slab is utilizedand the safety factor is many times higher so that this form

of construction must be most attractive in areas having alarge temperature range of, say, 7O0C or more.

(6) Because of the high resistance to buckling of the slab, it is nolonger imperative to restrict the maximum compressiveforce generated in the rails. The need for periodic restressingof the rail is eliminated and advantage can be taken toaccept a stress-free condition at a lower temperature therebyreducing the likelihood of rail fractures during cold weather.

(7) Slab track, more especially continuous slab track, offers thepossibility of continuous support to the rail. Even if the railis supported on a series of discrete pads, the support is morestable and the elimination of the possibility of voids atsupport positions means that the rails are subjected to muchlower bending stresses than those which they are designed toresist. There are grounds, therefore, for redesigning rails foruse on slab track either to use a lighter section or to transfermore metal into the head to provide for extra wear beforereplacement.

Although techniques are available to restore the track geometryof slab track after subsidence of the slab, it is both difficult andcostly in time and money to do so. It is far more preferable, andalways possible, to carry out the construction of slab'track,including its foundations, in such a way that the likelihood ofpermanent slab movement is very low.

The likely cost of installing slab track depends on the type ofdesign adopted, but generally the cost of site preparation up tothe sub-ballast level would be much the same as for conven-tional track whereas the provision of the remainder is likely tobe 25 to 75% more than for conventional high-quality CWRtrack. However, this may be offset, and even reversed in manycases, by the reduced construction depth of the slab track whichwill reduce the ancillary operations, e.g. new tunnels can bedriven with smaller cross-sections; overhead electrificationclearances can be achieved without expensive bridge raising orformation lowering.

The laying of slab track is much slower than that of conven-tional track. The system which uses precast units surrounded byin situ concrete would typically be laid at about one-third of therate of conventional track. The provision of a continuous slabmay well be slower still, with the complication that the workcannot be subdivided and carried out in a series of short trackpossessions.

The main difference in logistics of supplying materials for slabtrack compared to conventional track is the need to placeconcrete soon after it has been mixed. However, this is only aproblem of site organization and even in remote situations, suchas in long tunnels, has not caused major problems.

25.9.1 Prefabricated slab track

Prefabricated slab tracks have been installed in a number ofparts of the world. The major differences between these slabsand those described above are:

(1) Prefabricated systems are designed for ease of replacementof units during short possessions so that, except for the rail,there is little connection between adjacent units. Conse-quently, there is a much greater reliance on a strong,accurate formation.

(2) In practice, the installations using this type of system havesuffered from the effects of dynamic forces generated as awheel has passed over the junction between adjacent units.The need for maintenance of track geometry on a regularand frequent basis is usually encountered. This is oftenprovided by the introduction or removal of height-adjustingpads located under baseplates mounted on the units.

Figure 25.11 Section of slab track laid in tunnel (to increaseheadroom for electrification, and to reduce maintenance costs)

Although these systems place a greater need on periodmaintenance than do the more continuous forms of slab track,the ease and lower cost of adjusting the track following majormovement of the slab more than compensate for this extra effortin certain locations. In such locations, where the stability of slabtrack is required to allow high-speed traffic or the use of reducedclearances but where there is a strong possibility of significantmovement of the track, e.g. as a result of an earthquake, thisform of slab track construction has many advantages overothers.

Estimates of the limit of speed at which trains can run onconventional tracks vary between about 250 and 400 km/h. Thelimits that are placed on speeds of trains by slab track are largelythose of accuracy of measurement in order to detect and correctmisalignments at the construction stage. If the call for higherspeeds continues to be made, it may only be possible to meet itby using slab track.

Similarly, if the need to provide sinuous railway routesthrough highly congested areas continues, the use of slab trackshould allow this to be done using the least amount of space andwith the least constraint on train speeds.

25.10 Track maintenance and renewal

Generally, the greater the strength and inherent stability of thedesign of the track construction and its foundation, the less

demanding is the maintenance task. The stiffness of the rail, thesleeper spacing and the depth of ballast are important factorsaffecting the cost of maintenance.

The evolution of the automatic levelling lining and tampingmachine in the 1960s (Figure 25.12) changed the general patternof track maintenance. The original manual method of mainten-ance, using beater picks, was to drive individual pieces of ballastunder the sleeper to obtain the required rail level and firmness ofsupport. A later development, pioneered in France, was to jackthe sleeper and spread small stone chippings over the bearingarea. This system was known as measured shovel packing(MSP) and was extremely successful in producing good trackalthough expensive in labour.

A recent development has been the mechanization of thisprocess using a machine known as a 'Stoneblower'. Thismachine automatically measures the voids under the sleepersand, using sophisticated electronic controls, pneumatically in-jects the correct quantity of stone to produce the correctlongitudinal, vertical track profile. A prototype machine usingthis process has recently been constructed (Figure 25.13). Deve-lopment work on this principle has shown that track maintainedby this system only deteriorates at approximately one-quarter ofthe rate of track maintained by conventional tamping. Veryconsiderable savings are therefore possible in maintenance costsby using this new technique.

Other machines used in association with tamping and stone-blowing are ballast-regulating machines for positioning ballast

Figure 25.12 Tamping and lining machine suitable for use onswitches and crossings. (Courtesy. Plasser & Theurer)

and providing the correct ballast profile and, for cases whereballast requires cleaning and renewal, ballast-cleaning machineswhich screen dirty ballast, returning usable stone and rejectingdirt which has accumulated over the years. Ballast cleaning isessential for maintaining track stability as it is vital to ensuregood natural drainage in the ballast bed; this is particularlyimportant in heavily used lines with high annual tonnages andaxle loads.

With the reduction in manpower and greater reliance beingplaced on machines, methods additional to visual inspection bytrack walkers are necessary to ensure that track standards aremaintained. This is achieved by the use of track geometryrecording cars. These produce graphical records of the right-and left-hand top, track gauge variations, twist (the mutualangular deflection between two axles on a rigid frame), right-and left-hand versines and a location line showing referencepoints such as mileposts, bridges, stations, switches and cross-ings, etc. The data accumulated are recorded on an on-boardcomputer and the information is presented in a printout andalso, if required, in graphical form. This information is used toallocate priority to maintenance programmes, including tamp-ing and lining of the track and also other remedial work.

A problem that has increased in recent years is rail corruga-tion. The cause of corrugation is not yet known but considerableresearch is being done by various academic and railway organi-zations on this subject. Meantime, civil engineers are faced withthe problem of removing such corrugations. They occur both as

shortwave corrugations of 40 to 18Om frequency with a depthof 0.1 to 0.4mm and also as longwave corrugations of 200 to3000 mm in frequency, with depths up to 3 mm.

If corrugation is not removed from the running surface,defects occur which can lead to metal fatigue. As a result, railsrequire replacing prematurely to avoid the risk of fractures. Theextra vibration induced by corrugation causes the fasteningelements between rails and sleepers to become loose, leading toexcessive wear and early failure. The additional forces imposedby corrugation also increase the strain on sleepers and ballastcausing deterioration and premature replacement. Corrugationgenerates noise, and in densely populated areas the result is anunacceptably high level of aggressive noise for local residents. Itis also contrary to the aim of achieving increased standards ofcomfort for rail passengers.

Corrugations can be removed by means of rail grinding, andspecial trains operated by Messrs Speno provide this serviceworldwide. Grinding is carried out with the flat ends of rotatingannular abrasive wheels and the trains operate groups ofgrinding wheels set at different angles over the running surfaceof the rails. These wheels are applied to the rail head and, after anumber of passes at 5/7km/h, a new rail head profile isproduced eliminating the corrugation in the process. Varioussizes of trains are in service ranging from small machines withsixteen grinding wheels to large trains operating more than ahundred. Recent research has highlighted the importance ofmaintaining the correct wheel-rail interaction involving both

Figure 25.13 Prototype 'Stoneblower' for packing and aligningtrack. (Courtesy: Plasser (GB) Ltd)

Figure 25.14 Track-laying gantries for use on single lines.(Courtesy: Geismar)

wheel and rail profiles. Rails often wear unevenly, particularlyon curves, and past practice has been to transpose such railsonce sidewear becomes excessive.

With speeds of modern passenger trains increasing, transpos-ing of rails is not now an acceptable practice as the transposedrail usually presents a sharp edge to the wheel; this results inpoor-quality running. It has therefore been necessary to developan on-track rail planing machine which has the capacity toreprofile the rail head and thus provide an acceptable wheel-railinteraction suitable for high-speed running.

Mechanization of track-relaying has been developed overmany years and present practice consists of laying sleepers bymeans of gantries and beams which carry up to sixty sleepers.These place the sleepers on the ballast and thereafter the rails(jointed or long-welded) are installed using further specialistmachines. On new construction, it is possible to lay 3 to 4 km oftrack per day but this is dependent upon good organization forthe supply of materials on a regular basis. On existing railways,different constraints apply, particularly relating to the passagetraffic on adjoining lines. Speed of relaying on such tracks ismuch slower but can easily exceed 2 km per day if large relayingmachines are used.

25.11 Railway structures

In the UK, all railway construction is subject to the approval ofthe Railway Inspectorate of the Department of Transport onbehalf of the Secretary of State for Transport. Works of newconstruction, reconstruction, alteration or addition are accord-ingly governed by Requirements and Recommendations laiddown by that Inspectorate; these are currently undergoingcomplete revision and metrication but the only section so farissued in the revised form is that relating to structural andelectrical clearances. The following paragraphs detail the mainrequirements, being based as appropriate on the undernotedpublications of Her Majesty's Stationery Office and such direc-tives as have been given by the Railway Inspectorate since theirissue: Railway construction and operation: requirements for pas-senger lines and recommendations for goods lines (1963 reprint),and Railway construction and operation and structural and

electrical clearances (1977). Where dimensions are given in thefirst of the above documents in imperial units, these have beenconverted to (rounded) S.I. units for the purposes of this chapter.

The Requirements are subject to some variations and relaxa-tions in the case of light railways and lines of local interest andthere are some modifications in the case of underground tuberailways.

None of the Requirements is necessarily applicable in retro-spect to existing railway structures.

25.11.1 General

All design, materials and standards of workmanship shouldaccord with the appropriate current British Standard or Code ofPractice, of which the most generally used are listed at the end ofthis chapter.

Where any of the Requirements cannot reasonably be met,the special dispensation of the Railway Inspectorate is necessaryand may well be conditional upon additional stipulations.

25.11.2 Stations

The lines leading to passenger platforms should be arranged sothat platform roads may be entered in the normal direction ofmovement without reversing.

Curvature of platform lines, and of station yards generally,should be minimized. Tracks through a station or serving asiding should not normally be on a gradient steeper than 1:260.

Platforms should be long enough to accommodate the longestpassenger trains serving them and have a non-slip surface. Theclear width of any platform throughout its length should not beless than 2 m; at important stations and for island platforms, itshould be at least 4 m over the greater part of the length, butmay be narrowed towards the ends to not less than 2 m. Thedescent at the ends of platforms is to be by ramps with agradient not steeper than 1:8.

Columns for the support of roofs, together with other fixedworks such as station buildings and passenger facilities, mustnot extend to closer than 2 m from the platform edge. A generalclear headway of at least 2.5 m is required over platforms.

Platform levels depend upon the floor heights of passengerrolling stock using them, so that the gap between them is assmall as practicable. In any case, the stepping distances betweenthe platform edge and the footboards of the passenger rollingstock should not exceed 275 mm laterally and 250 mm vertically.At platform edges, coping should overhang the face walls by300 mm and the recess so formed should be kept clear, so far aspossible, of permanent obstruction.

Where a hazard exists at the rear of a platform or around thesides of stairwells or subway ramps, suitable protective fencingmust be provided.

All station premises used by passengers or staff during thehours of darkness must be adequately illuminated, the minimumlevel of lighting on an open platform being 4 Ix. Station namesshould be conspicuously displayed at intervals along platformsand illuminated at night.

Where passengers must cross the lines to reach a platform, afootbridge or subway should normally be provided.

Stairways and ramps must be of adequate width to avoidovercrowding (but not less than 1 m wide) and at no pointnarrower than at the top or contracted by any fixed obstruction.The steps of stairs should not be less than 280 mm in the treadnor more than 180 mm in the rise (with optimum dimensions of300 and 150 mm respectively). Inclined slopes and ramps are notto be steeper than 1:8 (7.12°) while the angle of inclination ofescalators should not exceed 30° to the horizontal. Intermediatelandings should be provided between fixed flights of steps, withno single flight exceeding 3 m overall rise.

Stations should be constructed to be as fire resistant aspracticable and extinguishing equipment provided for dealingwith fires in station premises.

25.11.3 Bridges and viaducts

Bridges and viaducts may be constructed in steel, concrete(reinforced or prestressed), brickwork or masonry. The use ofother materials such as light alloys will be considered on theirmerits, but cast iron must not be used in any portion of thestructure of a bridge carrying a railway except when subject todirect compression only. Structural timber is usually restrictedto light overline bridges.

New and reconstructed bridges must be built to provide atleast the mandatory clearances to the kinematic envelope (des-cribed below) as illustrated in Figure 25.15. For British Rail,this requirement gives a standard structure gauge 4.640mmhigh (above rail level) and 2.340mm clear laterally from thecentreline of the nearest track (when the latter is straight;additional lateral clearance is necessary alongside curvedtracks).

Underline bridges must be provided with robust kerbs as partof the bridge structure and extending to a height of at least305 mm above rail level to contain the wheels of any derailedvehicle. Such bridges must provide for a safe lineside walkway,with a substantial parapet or railing not less than 1.250m inheight above the walkway (unless the main girders themselvesprovide a sufficient parapet). On bridges and viaducts which arelonger than 40 m or where visibility of approaching trains islimited, it may be necessary to incorporate recesses in theparapet design to afford the refuges described below.

Bridges and other structures built over or immediately adjac-ent to railway lines must be protected against the consequencesof their supports being struck by railway vehicles in the event ofa derailment. Such supports should be located as far from thetracks as practicable. If they can be kept 4.5 m or more clear ofthe nearest rail, there will normally be no requirement to designfor impact forces. Where this clearance cannot be achieved, thesupports must be designed with a degree of robustness andcontinuity to minimize the effects of contact by a derailedvehicle and to withstand nominal impact forces.

Overline bridges and elevated roads alongside railways mustbe provided with parapets complying with Department ofTransport Technical Memorandum (Bridges) No. BE 5. Theseparapets are intended to protect pedestrians and/or to containerrant vehicles and prevent them from falling on to the railway;they may also be required to be solid, to prevent splash, reducenoise or screen railway electrification equipment.

For bridges carrying roads, such as motorways, from whichpedestrians, animals and pedal cycles are excluded by order, theminimum height of parapet above the adjoining paved surfacemust be 1.250 m. The lower 600 mm of the parapet should havea mesh or solid infill panel, which may be mounted outside anylongitudinal members.

For bridges carrying all-purpose roads and footbridges, theparapet is to have a traffic face which is smooth and withouthand- or footholds. The minimum height of parapet above theadjoining paved surface must be 1.500m. For bridges usedfrequently by equestrians, the height must be at least 1.800m.

If a bridge over a railway which is electrified on the overheadsystem has a parapet with a plinth on its outer face, access mustbe denied to that plinth. Any parapet more than 100 mm thick isto be surmounted by a 'steeple-shaped' coping (i.e. with steepsides meeting at the top at an acute angle).

At locations where the likelihood of vehicle impact with theparapet and consequential damage resulting from its penetra-tion outweigh the hazards resulting from the containment andredirection of errant vehicles within the traffic scheme, 'highcontainment' parapets must be provided.

Pipelines carrying liquids or gases over the railway must beincorporated in a bridge constructed for another purpose or besupported by a purpose-designed beam or service bridge. Such abridge should preferably span the railway without intermediatesupports. Consideration may be given to a free-standing designof pipe crossing only in the case of low-pressure water mains orsimilar pipes conveying non-hazardous materials.

The design loadings and other criteria for both underline andoverline bridges are generally as set out in BS 5400. The majorrailway undertakings in Britain require underline bridges to bedesigned and constructed by their own staff or under their directcontrol.

Where bridges are to be constructed under or over existingrailways, special techniques are usually necessary to minimize

Standard structure gauge

Kinematic Envelope

Nearest face of signal posts and otherisolated structures less than 2 m inlength but excluding masts carryingoverhead line equipment on electrifiedrailways. [Nearest face of all other structuresincluding masts carrying overhead lineequipment on electrified railways.

Figure 25.15 Department of Transport-standard structure gaugeand kinematic load gauge

Standard dimension between structuresfor double line standard gauge railway

Columns and otherfixed works on platforms

Designed Rail LevelRecess for signal wires and cables if requiredThis space to be kept clear as faras possible of permanent obstructionsCentre Line of Track Centre Line of Track

NominalStandard Gauge

Maximum width of rollingstock over all projections

NormalMinimum (new lines)Minimum(existing lines)

Clearance between kinematicenvelope and railway operationalstructures above cantrail level.Normal — 250. Minimum — 100.On railways with existing orproposed overhead electrificationthe requirements for electrical

clearance, wherethese are greater,shall apply

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the interruption to the normal operation of such railways; therequirements of these techniques may significantly influence notonly the temporary works but also the design of the permanentworks. It is usually more economic to ensure that work adjacentto running lines, particularly excavation, is located sufficientlyfar away from them to avoid the necessity for the imposition ofspeed restrictions on railway traffic. The design and construc-tion of all bridges and viaducts should also be such as tominimize future maintenance on grounds of both economy andavoidance of interference with railway traffic. Provision shouldbe made to ensure that all parts of bridges are accessible forregular inspection and maintenance and that all exposed steelcan be painted. Underline bridges should be designed to carrynormal ballasted track.

25.11.4 Clearances

The structural and electrical clearances (to be provided on newlines and on existing railways where new structures are built orexisting structures are modified or where clearances are other-wise altered) are specified in the revised part of the above-mentioned Department of Transport Requirements publishedin 1977.

Structural clearances are related to the 'kinematic envelope'of the stock which will use the line and are illustrated in Figure25.14 in respect of lines carrying trains at speeds up to 200 km/h;that figure is based for the purpose of illustration on thekinematic envelope applicable to British Rail vehicles.

The kinematic envelope is derived from the static load gaugeas follows: (1) the maximum permitted cross-sectional dimen-sions of vehicles and their loads when at rest and locatedcentrally on straight and level track; (2) the static load gaugemust be enlarged to allow for the maximum possible displace-ment of the vehicles when at rest or in motion, with respect tothe rails, taking account of their suspension characteristics andmaking allowance for maximum permitted tolerances in themanufacture and maintenance of the vehicles, including wear;and (3) the resultant kinematic load gauge must then be furtherenlarged to take account of the maximum permitted tolerancesin gauge, alignment, top- and cross-level of track including theeffects of wear. The kinematic envelope so derived thus containsthe full cross-section of vehicles and their loads under anypermissible condition of operation and maintenance of bothvehicles and of track. It does not, however, allow for the end-throw and centre-throw of vehicles on curved track, and addi-tional allowance for these must be made as appropriate.

Electrical clearances are dependent upon the nominal voltageof the overhead conductor system and upon whether the cate-gory of clearance is to be normal, reduced or special reduced.They must include provision for tolerances in the installationand maintenance of the overhead equipment and of the tracklimiting values of the vertical clearance for normal, reduced andspecial reduced clearance conditions between the underside of astructure and the kinematic load gauge shown diagrammaticallyin Figure 25.15 (for 25 kV a.c.). Also shown are the corre-sponding clearances between the undersides of structures andthe designed rail level, based on the normal British Rail staticload gauge. It will be noted that the standard structure gauge onBritish Rail allows for overhead electrification at voltages up to25 kV, albeit with 'close tolerances'; the vertical distance fromrail level to the undersides of structures should ideally beincreased to 4.780 m if this can be achieved with reasonableeconomy, and further increased where a bridge is adjacent to alevel crossing, over which the overhead equipment has to beraised to give 5.600 m clearance over road levels.

25.11.5 Refuges

Refuges are to be provided in all tunnels and also on longbridges and viaducts and where the railway is enclosed bylineside structures unless lateral clearances over a continuouslength of 40 m or more are such that staff can stand in safetyduring the passage of trains. Similarly, on embankments and incuttings, wherever lineside clearances over a length of 40 m ormore do not give safe standing room, refuge spaces are to beprovided. The criteria as to what clearance allows men to standat the lineside in safety are defined as 830mm from thekinematic envelope at its widest point for train speeds of up to160 km/h and 1.700m for train speeds between 161 and 200km/h.

Refuges and refuge spaces should be located on both sides ofthe line at a spacing not exceeding 40 m, staggered to give aneffective spacing of 20 m or less. Refuges must be at least 2 mhigh, 1.400 m wide and 700 mm deep, with their floors substan-tially at the level of the adjacent lineside walkway. Handholds,to assist men in keeping their balance during the passage of atrain, are to be provided in refuges and also elsewhere if lateralclearances only marginally exceed the criteria specified in theprevious paragraph.

25.12 Inspection and maintenance ofstructures

In the interests of safety and economy, it is essential to have up-to-date and reliable information as to the current condition ofall railway structures and to carry out any necessary repairs orrenewals in due time.

The key to proper maintenance, therefore, lies in a system ofregular routine and methodical examination by suitably trainedand experienced staff in order to provide the responsible main-tenance engineer with well-founded records, for each structure,as to: (1) its condition; (2) its current fitness for its intendedpurpose; (3) its rate of deterioration; and (4) the urgency forremedial action.

It is common practice to legislate for superficial examinations,undertaken at intervals of about 1 year, and less frequentdetailed examinations, in which every element of the structure iscarefully and thoroughly inspected at close range and the extentof any deterioration quantified. The frequency of detailedexaminations will depend upon the nature, age and condition ofthe structure, but the interval between such examinationsshould rarely exceed 6 years unless there are special problems orunjustifiably high costs in providing adequate access; in thelatter event, decisions as to the action to be taken following anexamination should have particular regard to the length of timelikely to elapse before the next one. Additional special examina-tions may be necessary after flooding, damage from impact, fireor vandalism and similar eventualities.

Proper examination depends upon adequate safe access to allparts of a structure. Suitable provision should have been madefor this in the initial design by providing removable panels,walkways, fixing points for ladders or safety lines and byavoiding the creation of inaccessible spaces. Where the latter hasnot been done, some 'opening-out' should be undertaken, atleast on a sampling basis. Recourse to expensive scaffolding mayoften be avoided by the use of mobile inspection platforms.

Examiners must compile full written reports of detailedexaminations. To facilitate this, most railway undertakings usespecially devised forms, which act as a checklist and encourageclear, concise and comprehensive coverage. Sketches or photo-graphs are valuable for illustrating defects. Reports must recorddefects in quantitative form, e.g. the loss of section (or the

amount of section remaining), the extent of any displacement,the magnitude of any movement, the width, length and orien-tation of cracks, the extent of hollow-sounding brickwork, thedepth of perished mortar or the degree of water percolation.Where appropriate, 'tell tales' and plumbing/levelling pointsshould be established and readings from them recorded. In somecases, more sophisticated techniques of investigation may becalled for. The objective should be to ensure that the mainten-ance engineer has readily available data as to the nature, rateand extent of the development of any defect, i.e. the way inwhich deterioration is occurring over a period of time bycomparing a succession of reports. Thus, he is assisted indiagnosing the cause of trouble and guided in his judgement asto the urgency for remedial action.

Financial constraints may well limit the number of structureswhich can receive maintenance attention in any year, and it istherefore necessary to establish some kind of priority ratingsystem for defects, to ensure that available resources are dir-ected where the need is most urgent. This may well involve someassessment of the residual strength of members. Calculationsand possibly instrumental tests may be necessary to determinecritical stresses; these should be based on the nature andmagnitude of the loads actually having to be sustained andshould have regard to the current condition of members,allowing for loss of original section, distortion and similardefects.

The nature, extent and timing of any repair will vary widelydepending on the circumstances applicable in individual cases.Decisions hereon must rely heavily on the informed judgementof the experienced maintenance engineer. For repairs to beeffective, it is essential that the cause of the defect is correctlydiagnosed and that that cause, rather than its results, is properlydealt with. Desirably, this should be done before serious defectsdevelop, by preventive maintenance of a stitch-in-time nature,e.g. painting of metalwork and timber, pointing of brickworkand masonry and sealing points of water percolation.

It is also a matter of professional judgement to determinewhen repairs are no longer economic and reconstruction isjustified.

25.13 Research, development andinternational collaboration

The principal railway co-ordinating body in Europe is the UIC(Union Internationale des Chemins de Fer) whose headquartersis in Paris. Its primary purpose is the encouragement anddevelopment of standard railway policies and practices through-out Europe. It has a permanent secretariat but it works throughcommittees covering the activities of most railway departments.Twenty-six nations are constituent members.

Its technical research co-ordinating arm is ORE (Office forResearch and Experiments) which has its office at Utrecht. TheUIC issues specifications, standards, and recommendations forrailway practice to which railway administrations are expectedor encouraged to adhere. The ORE is represented on alltechnical committees of UIC and issues reports made by work-ing parties or by specially commissioned persons of academic orprofessional distinction to member administrations of UIC.

The field and laboratory work of ORE is assigned to memberadministrations or to universities or technical schools; suchwork is usually progressed by an appropriate working partycomposed of delegates from member administrations servicedby a technical secretary from ORE. Distinguished men fromindustry and universities are sometimes co-opted to serve onspecific working parties; UIC and ORE reports are normallyissued simultaneously in English, French and German, and

these languages are used at UIC and ORE meetings, Frenchbeing the primary procedural tongue.

Another European body which makes a significant contribu-tion to railway technical literature in its periodical bulletins, isthe International Railway Congress Association (IRCA) whoseadministration is in Brussels.

The principal technical railway co-ordinating organization inthe US is the American Railway Engineering Association(AREA) of Chicago which issues a periodical journal coveringminutes of meetings, reports of special committees and recom-mendations for specifications and technical procedures. TheAREA also sponsors research and development projects whichare mainly progressed by the Association of American Rail-roads (AAR) at its Research Centre at Chicago. The PanAmerican Railway Congress is a similar but less developedorganization covering the countries of South America. Muchvaluable railway research and development work is done inJapan and the English version of the monthly Journal of thePermanent Way Society of Japan, founded in 1958 in Tokyo,provides a comprehensive contemporary reflection of this work.

Important permanent way research centres also exist inPrague, Moscow, Belgrade and Johannesburg but the results oftheir activities are normally limited to domestic distribution.The chief technical and development railway work carried out inthe UK is associated with the Research and DevelopmentCentre at Derby which has a considerable international repu-tation. Its reports are generally limited to internal circulationwithin the British Railways Board but most of its railwayresearch projects are reported through papers and articles forthe British engineering institutions, chiefly the Institution ofCivil Engineers, the Institution of Electrical Engineers, theInstitution of Mechanical Engineers, and the technical press. Italso accepts commissions for research from outside the BritishRailways Board.

The national railways of the UK, Canada, France, Germanyand Japan are also associated with sponsored railway consul-tancy organizations which, whilst primarily concerned with thetechniques of railway organization, also offer technical advice toclients.

Contemporary railway civil engineering practice and develop-ments in the UK and elsewhere are more to be found in paperspresented to the railway division of the Institution of CivilEngineers and to the Permanent Way Institution, in the bulle-tins and journals of ORE, IRC, and the AREA and in therailway technical press of the UK, France, Germany andAmerica, rather than in the few standard textbooks on railwayengineering which are necessarily less up to date.

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BS 882:1983'Specification for aggregates from natural sources for concrete/

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BS 1377:1975'Methods of test for soil for civil engineering purposes/

BS 2589:1955 (1984)'Railway track spanners/

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BS 5268:1984'Code of practice for the structural use of timber/

BS 5400:1978 Parts 1 to 10'Steel, concrete and composite bridges/

BS 5493:1977 (Amended 1984)'Code of practice for protective coating of iron and steel structuresagainst corrosion/

BS 5930:1981'Code of practice for site investigations.'

BS 6031:1981 (Amended 1983)'Code of practice for earthworks/

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CP 2004:1972 (Amended 1975)'Foundations/