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Permanent way
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The permanent way means the physical elements of the railway line itself: generally the pairs of rails typically laid on sleepers ("crossties" or just "ties", in North America) embedded in ballast, intended to carry the ordinary trains of a railway. This page describes British practice and British terminology, both of which diverge significantly from the later North American and other usages.
It is described as permanent way because in the earlier days of railway construction, contractors often laid a temporary track to transport spoil and materials about the site; when this work was substantially completed, the temporary track was taken up and the permanent way installed.
Configuration
Permanent way is the generic term for the track rails, sleepers and ballast on which railway trains run.
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Encyclopedia
The permanent way means the physical elements of the railway line itself: generally the pairs of rails typically laid on sleepers ("crossties" or just "ties", in North America) embedded in ballast, intended to carry the ordinary trains of a railway. This page describes British practice and British terminology, both of which diverge significantly from the later North American and other usages.
It is described as permanent way because in the earlier days of railway construction, contractors often laid a temporary track to transport spoil and materials about the site; when this work was substantially completed, the temporary track was taken up and the permanent way installed.
Configuration
Permanent way is the generic term for the track rails, sleepers and ballast on which railway trains run. British practice has diverged quite sharply from that in North America and continental Europe. Although the configuration of the track today would be recognised by engineers of the 19th century, it has developed significantly over the years as technological improvements became available, and as the demands of train operation increased.
However the traditional form consists of:
- two parallel iron or steel rails, a fixed distance apart, on which the wheels of trains run,
- transverse beams called sleepers, set at a close spacing, that maintain the specified spacing of the rails and that distribute the concentrated loading of train wheels,
- fastenings to hold the rails and sleepers together,
- a layer of mineral ballast placed under and around the sleepers, to further distribute the train loading, and to resist lateral displacement of the sleepers.
The spacing between the rails is referred to as the gauge, always measured between the inner faces of the rails. The original soil below the ballast (or a strengthened substitute) is called the formation.
Present-day practice
The current form of permanent way in Britain has stabilised on continuously-welded flat-bottom rails laid on heavy prestressed concrete sleepers, supported on a thick layer of crushed stone ballast. The rail section currently being installed is the UIC60 flat-bottom section.
However a very large volume of 113A section is in place and will continue to carry trains for some long time. Typical service life for track is 15 30 years, but wide variations are encountered, so a great deal of track is in use today, that was installed in conformity to earlier standards, and some pre-war equipment (made before 1939) can easily be found in sidings.
The pre-stressed concrete sleepers have malleable iron inserts cast in during manufacture, to accept resilient fastenings to hold the rails down; the proprietary Pandrol design predominates hugely, but significant lengths of a Vossloh proprietary fastening can also be found. As well as receiving the fastenings, the inserts prevent lateral movement of the rail, and therefore retain the track gauge.
The rail has a plastic pad under it at the sleeper positions, and this pad is important in preventing the transmission of high frequency vibration into the sleepers and ballast. Small plastic insulators are provided between the rail foot and the shoulders of the inserts for the fastenings.
The ballast must be clean (free of fine material) at the time of installation and be angular and of a specified, fairly large particle size. It is normally installed in a thickness of 300 mm or more. Where the formation is not of good quality material, a blanket layer may be installed below the ballast, consisting of sand and a geotextile layer, to prevent migration of the ballast particles into the formation.
The formation should normally be graded so as to slope to the cess the area at the edge of the track made at a lower level, to act as a sump for surface water. A cess drain may be installed where necessary, and efficient removal of rainwater is essential.
Slab track
An important alternative to the standard track type described above is slab track, sometimes referred to as ballast-less track. In this variant the sleepers and ballast are replaced by a concrete (or asphalt) slab. A number of alternative types have been adopted, including
- Ladder track, in which the rails are supported on pre-cast concrete longitudinal bearers with concrete transoms to keep gauge;
- A continuously placed in-situ slab in which the fastening inserts are formed before the slab concrete cures (known as the PACT system).
- Cast-in sleeper track, in which concrete sleepers are fixed in an in-situ concrete slab. The sleepers in this system usually have protruding reinforcing bars extending into the in-situ concrete to provide a shear key.
Installing slab track is generally more expensive than ballasted track and it requires lengthy track possessions to install it properly (some skimped installations have failed in service and they proved very difficult and expensive to rectify), but maintenance costs are reduced drastically and the life time of the structure is significantly larger. In consequence slab track has not found widespread adoption in Britain, and the relatively few examples are installed in special-case situations.
Historical development of track
The earliest use of a railway track seems to have been in connection with mining activities in the 17th century; round timber rails could be put down to enable individual carts carrying heavy mineral ore to be pushed along it by manpower. Variants of this used squared timber rails offering a flat top surface: if the rails were wide enough a cart could be pushed along it manually. Obviously the difficulty was keeping the carts on the rails, and the carts needed continuous guidance by an attendant.
Wear of the timber rails was also a problem, and when wrought iron became available, iron plates were laid on the top of them. A major leap forward took place when the plates were made L-shaped in cross section, with the raised lip keeping the wheels in alignment. For the first time something resembling a railway was possible, because a horse could pull more than a single wagon a train without the necessity to steer the individual vehicles. The guidance was rather approximate, but as no great speed was attempted this did not matter. This format was called a plate-way, from the metal plates, and found surprisingly wide adoption in Britain. Junctions were possible, as were level crossings with roads in fact in some cases the plate-ways ran in the highway. The L-shaped plates were used either fully supported on timber bearers and spanning between sleepers (usually stone blocks) as the case may be.
Edge rails
As locomotives began to be developed, it was obvious that much greater loads needed to be supported, and the thin wrought iron plates did not provide enough support, nor a sufficiently accurate guidance, and the next major advance was the use of edge rails, in which the metal rails were deeper than their width, providing much greater strength; and the guidance was now achieved by flanges on the wheels, instead of on the rails. The rails did not have a flat base, and were held in jaws in a metal support, which, in one form or another, is recognisable as a chair.
This system was instantly successful, although some false starts took place. Some early rails were made in a T cross section, but the lack of metal at the foot limited the bending strength of the rail, which has to act as a beam between supports. The rails were at first made of cast iron in lengths of typically three feet, spanning between stone blocks. Cast iron has a poor strength in tension, and at first these short lengths were the longest that could be cast. Later it was possible to cast longer lengths, and the fish-bellied profile was perpetuated over several support positions.
The stone blocks had been assumed to be permanent, but experience quickly showed that they settled and gradually moved under traffic, creating chaotic track geometry and causing derailments.
As metal technologies improved, it became possible to make rails of wrought iron, and progressively somewhat longer, and with a heavier, and therefore stronger, cross-section. By providing more metal in the foot of the rail, a stronger beam was created, achieving much better strength and stiffness, and a section was created similar to the bullhead rail section still visible today.
At first it was almost symmetrical top-to-bottom, and was described as a double-headed rail. It may be that the intention was to invert the rail after the top surface had become worn, but rails tend to develop chair gall, an attrition of the rail where it is supported in the chairs, and this would have made running on the former bottom surface impossibly noisy and irregular. It was obviously better to provide the extra metal on the top surface and gain extra wear there without the need to invert the rail at half life.
Flat bottom rails were employed on some early railways; they had a somewhat wider foot than we see today, and it appears that the intention was that they would be dogged down direct to timber sleepers. Indenting of the sleeper was the problem, obviously, and this form was not adopted on any widespread basis in Britain until after World War II.
Sleepers
The next development was to provide timber sleepers, that is transverse beams supporting the two rails that form the track, replacing the individual stone blocks formerly used. This system has the major advantage that maintenance adjustments to the track did not disrupt the all-important track gauge. The alignment of the track could be adjusted by sluing it bodily, without loss of gauge.
By now we had relatively long (perhaps 20ft) wrought iron rails supported in chairs on timber cross-sleepers a track form recognisable today in older track.
Steel sleepers were tried on a limited basis in the period before WW II, but were generally unsuccessful, probably due to the use of thin sections that corroded. The British Steel Company used a flat-bottom design in their private sidings, but they did not come into common use until about 1995. Their dominant usage now is for life extension of existing track on secondary routes. They have a significant advantage on weak formations and poor ballast conditions, as the bearing area is at a high level, immediately under the rail seat.
Ballast
The track was originally laid direct on the ground, but this quickly proved unsatisfactory and some form of ballast was essential, to spread the load and to retain the track in its proper position. The natural ground is rarely strong enough to accept the loading from locomotives without excessive settlement, and a layer of ballast under the sleeper reduces the bearing pressure on the ground. The ballast surrounding the sleepers also tends to keep them in place and resists displacement.
The ballast was usually some locally available mineral product, such as gravel or reject material from coal and iron mining activities. The Great North of Scotland Railway used river gravel round pebbles which must have made track maintenance a challenge. In later years the ash from steam engines was used (as it would otherwise have incurred a cost to dispose of it) and slag (a by-product of steel making).
Early track gauges
The early railways were almost exclusively local concerns involved with conveying minerals to some waterway; for them the gauge of the track was adopted to suit the wagons intended to be used, and it was typically in the range 4 ft to 4ft 8½ in, and at first there was no idea of the need for any conformity with the gauge of other lines. When the first public railways developed, George Stephenson's skillful innovation meant that his railways were dominant and the gauge he used was therefore the most widespread. As early notions of linking up different railway systems evolved, this gauge secured general adoption. It is more or less an accident of history that this gauge which suited the wagons already in use at the colliery where George Stephenson had been an engine man became the British standard gauge: it was exported to most of Europe and North America.
Reference is sometimes made to the "gauge" of ruts in stone roadways at ancient sites such as Pompeii, and these are often asserted to be about the same as Stephenson's gauge. Of course the ruts were made by the wheels of carts, and the carts were of a sensible size for horse drawn carts prior to the industrial era, pretty much the same as the size of the pre-railway carts at the colliery where Stephenson worked: that is the only connection.
Broad gauge track
When Isambard Kingdom Brunel conceived the Great Western Railway (GWR), he sought an improved design for his railway track and accepted none of the previous received wisdom without challenge. The 4ft 8½in gauge had been fine for small mineral trucks on a horse-drawn tramway, but he wanted something more stable for his high speed railway. The large diameter wheels used in stage coaches gave better ride quality over rough ground, and Brunel originally intended to have his passenger carriages carried in the same way on large diameter wheels placed outside the bodies of the carriages. To achieve this he needed a wider track gauge and he settled on the famous 7ft broad gauge. (It was later eased to 7ft 0Όin). When the time came to build the passenger carriages, they were designed conventionally with smaller wheels under the bodies after all, but with a seven-foot track gauge the bodies could be much wider than on the standard gauge. His original intention to have the wheels outside the width of the bodies was abandoned.
Brunel also looked at novel track forms, and decided to use a continuously supported rail. Using longitudinal timbers under each rail, he achieved a smoother profile while not requiring such a strong rail section, and he used a shallow bridge rail for the purpose. The wider, flat foot also meant that the chair needed by the bullhead section could be dispensed with. The longitudinal timbers needed to be kept at the proper spacing to retain the gauge correctly, and Brunel achieved this by using timber transoms transverse spacers and iron tie-bars. The whole assembly was referred to as the baulk road railwaymen usually call their track a road. Initially Brunel had the track tied down to timber piles to prevent lateral movement and bounce, but he had overlooked the fact that the made ground on which his track was supported between piles would settle. The piles remained stable and the ground between them settled so that his track soon had an unpleasant undulation, and he had to have the piles severed, so that the track could settle more or less uniformly. A variant of the baulk road can still be seen today on many older under-bridges where no ballast was provided. The design varies considerably, but in many cases longitudinal timbers are supported directly on the cross-girders, with transoms and tiebars to retain the gauge, but of course with modern rails and base-plates or chairs. The longitudinal sleepers are somewhat similar to modern-day Tubular Modular Track.
The group of railways that had Brunel as their engineer were successful and the broad gauge track spread throughout the west of England, South Wales, and the West Midlands. But as the British railway network spread, the incompatibility of the two systems became a serious blockage, as a wagon could not be sent from one system to the other without transshipping the goods by hand. A Gauge Commission was appointed to determine national policy. The Broad Gauge was technically superior but conversion of the standard gauge routes to broad would have meant reconstructing every tunnel, bridge and station platform, whereas universal adoption of the standard gauge only required the progressive conversion of the track itself. The broad gauge was doomed, and no further independent broad gauge lines could be built.
The existing broad gauge routes could continue, but as they had no development potential it was only a matter of time before they were eventually converted to standard. In the meantime an extensive mileage of mixed gauge track was installed, where each line had three rails to accommodate trains of either gauge. There were some instances of mixed gauge trains being run, where wagons of each gauge were run in a single train. The legacy of the broad gauge can still be seen where there seems to be an unnecessarily wide space between station platforms.
1900 to 1945
At the beginning of the twentieth century, the form of track had converged on the use of wrought iron bullhead rails supported in cast iron chairs on timber sleepers, laid in some form of ballast. Many railways were using very light rails and as locomotive weights and speeds increased these were inadequate, so that on main lines the rails in use were made progressively heavier (and stronger). Metallurgical processes improved and better rails including some steel rails came into use. From a maintenance point of view the rail joints were the source of most of the work, and as steel-making techniques improved it became possible to roll steel rails of increased length reducing the number of joints per mile. The standard length became 30ft, then 45 ft and finally 60ft rails became the norm. For main line use the standard rail section became the 95BH section, weighing 95lb per yard. For secondary routes a lighter 85BH section was used.
Flat bottom rails were still seen as undesirable for main line railway use, despite their successful use in North America, although some lightly operated British railways used them, generally spiked direct to the sleepers. Under heavy usage they indent the sleepers severely and the incremental cost of a base-plate appeared at this early date, to rule the flat bottom section out.
Timber sleepers were expensive and not durable, and the railways engineers had strong and conflicting views about the best wood species and the best preservative treatments. Some railways experimented with steel sleepers in the 1920s but these were not successful, probably due to inadequate steel section. The railways moved towards standardisation on a softwood sleeper preserved by pressure injection of creosote, measuring 8ft 6in long by 10in by 5in. Chairs were secured to the sleepers by triennials (steel spikes driven through a timber sleeve) or three chair-screws on first class routes. The GWR alone among the main line railways kept to its own standard, the 00 rail at 97½ lb/yd, and with two chair-bolts securing each chair to the sleeper, with the head of the bolt under the sleeper and a nut above the chair -- more secure but much more difficult to adjust.
Some experiments were made before 1945 with reinforced concrete sleepers, in most cases with bullhead chairs mounted on them. This was in response to the very high price of the best (most durable) timber but reinforced concrete sleepers were never successful in main line use. Concrete pots were also used in sidings; they are sometimes called twin-block sleepers, and consisted of two concrete blocks each mounted with a chair, and an angle iron connecting them and retaining the gauge.
Post-war developments
At the end of the war in 1945, the railways were worn out, having been patched up following war damage without the availability of much new material. The country was economically in a weak situation also, and for nearly a decade after the war, materials especially steel and timber were in very short supply. Labour too was seriously restricted in availability.
The railway companies became persuaded that the traditional bullhead forms of track needed revision, and after some experimentation a new flat bottom rail format was adopted. The British Standard sections were unsuitable and a new profile, a 109 lb/yard rail, was made the new standard. In 60ft lengths, laid on steel baseplates on softwood sleepers, it was to be the universal standard. The fastenings were to be of a resilient steel type, and for secondary routes a 98 lb/yd rail was adopted. Regional variations still persisted, and hardwood sleepers and Mills clip fastenings were favoured on the Eastern Region, for example.
The new designs were successful, but they introduced many challenges, especially as the availability of experienced track maintenance staff became acutely difficult, and poorly maintained flat bottom track seemed more difficult to keep in good order than poorly maintained bullhead track. The greater stiffness of flat-bottom was an advantage but it tended to straighten out between the joints on curves; and flat bottoms rigidity led to high vertical impact forces at badly maintained joints and this resulted in high volumes of fatigue fractures at the joints. Moreover the elastic rail fastenings had little resistance to rail creep the propensity of the rails to move gradually in the direction of traffic, and the workload of pulling back the rails to regulate the joints was surprisingly high.
Long welded rails
Much of the work of maintaining the track was at the joints, especially as the stiff rails became dipped, and the joint sleepers took a hammering. Pre-war experiments with long welded rail lengths were built upon, and in the years from 1960 long rail lengths were installed, at first on hardwood sleepers but soon on concrete sleepers. In this pioneering stage some catastrophic mistakes in detailed design were made, but from about 1968 continuous welded rail became a reliable standard for universal installation on main and secondary routes. The form adopted used pre-stressed concrete sleepers and a 110A rail section a slight improvement on the 109 rails previously used the A was to distinguish it from the British Standard 110 lb/yd rail section, which was unsuitable. Rail fastenings eventually converged onto a proprietary spring clip made by the Pandrol company which was the exclusive form of fastening in Britain for about 30 years.
The welded track was to be laid on six to twelve inches of crushed stone ballast, although this was not always achieved, and the bearing capacity of the formation was not always taken into account, leading to some spectacular formation failures.
A further enhancement to the rail profile produced the 113A section which was the universal standard until about 1998; detail improvements to the sleepers and ballast profile completed the picture and the general form of the track had stabilised. This format is now in place over 99% of first-class main lines in Britain.
Track gauge
As stated, the general track gauge in Britain was . In the later 1950s general track maintenance standards deteriorated rapidly due to manning difficulties, and freight train speeds increased on some routes. Freight trains consisted almost entirely of short wheelbase (10ft) four-wheeled wagons carried on a very stiff elliptical leaf spring suspension, and these wagons showed an alarmingly rapid rate of increase of derailment events. Anyone standing at the lineside could watch a freight train pass at speed and observe several of the wagons weaving and swaying alarmingly even on good track, and derailment occurred when any poor track was encountered.
The dynamic behaviour of the wagons was the problem, but the solution adopted was to reduce the permitted speed of the wagons to 45 mph, and to reduce the track gauge by one-eighth of an inch, to 4ft 8?in (1432mm) for new installations of continuously welded track on concrete sleepers. Of course the long life cycle of the track meant that this conversion process would take 30 years or more to complete. However the basis of the gauge narrowing was mistaken. The idea seems to have been to reduce the free space for lateral movement of the wagons, so that they would be "contained" to run in a straight line. In fact railway vehicles are not contained by the flanges of the wheels except in very sharp curves, and in normal running the steering effect due to the conicity of the wheels is dominant. In reducing the track gauge the effective conicity is increased worsened and the tendency of the wagons to yaw and roll was increased. Many derailments took place on relatively new continuously welded rail track, and often such a derailment destroyed about a mile of the new track, as the freight train might take that distance to stop; the concrete sleepers were not robust under a derailed wagon's wheels.
The effect reduced as the wagon fleet was modernised (and other effects took first place) and the track gauge for new track was quietly restored to in 1998. Of course the vast majority of the track on main lines is still, as installed, at the tighter gauge, and it will be several decades before the gauge change is complete.
Switches and crossings
Terminology is difficult for "switches and crossings" (S&C) previously "points and crossings", or "fittings".
Early S&C allowed only a very slow speed on the subsidiary route (the "turnout"), so geometrical design was not too important. Many older s&c units had a loose joint at the heel so that the switch rail could turn to close to the stock rail or open from it. When the switch rail was closed a reasonable alignment was secured; when it was open, no wheel could run on it so it did not matter.
As speeds rose this was no longer feasible and the switch rails were fixed at the heel end, and their flexibility enabled the toe end to open and close. Manufacture of the switch rails was a complex process, and that of the crossings even more so. Speeds on the subsidiary route were rarely higher than 20 mph except in very special designs, and great ingenuity was employed to give a good ride to vehicles passing through at speed on the main line. A difficulty was the common crossing where continuous support to wheels passing was difficult, and the point rail was planed down to protect it from direct impact in the facing direction, so that a designed irregularity in support was introduced.
As faster speeds were required, more configurations of s&c were designed, and a very large number of components, each specific to only one type of s&c, was required. At faster speeds on the turnout road, the divergence from the main route is much more gradual, and therefore a very considerable length of planning of the switch rail is required.
About 1971 this trend was reversed with the so-called vertical s&c, in which the rails were held vertical, rather than at the customary 1 in 20 inclination. With other simplifications, this considerably reduced the stockholding required for a wide range of s&c speeds, although the vertical rail imposes a loss of the steering effect and the ride through new vertical s&c is often irregular.
CWR track
Continuously welded track was developed in response to the observation that the bulk of track maintenance work takes place at the joints: lengthening the rails from 20 to 30 to 45 to 60 feet progressively reduced the number of joints, and the logical extension of this would be to eliminate the joints altogether.
A major obstacle to doing so is thermal expansion: the rails expand in higher temperatures. Without joints, there is no room for the rails to expand; as the rails get warmer they will develop an enormous force in trying to expand. If prevented from expanding, they develop a force of 1.7 tonnes (17 kN) for every 1 degree Celsius of temperature change in a practical rail section.
If a small cube of metal is compressed between the jaws of a press, it will contract -- that is it will be squashed somewhat -- and a very large force can be resisted by it without ultimate failure. However if a long piece of metal of the same cross section is compressed, it will deform sideways into a bow shape; the process is called buckling, and the force it can withstand is very much less.
If the long thin piece of metal could be constrained to prevent it from buckling (e.g. by being contained inside a tube) then it can resist a much higher force. Obviously then if the rails can be constrained in a similar way, they can be prevented from buckling. The weight of the track resists buckling upwards, so buckling is most likely to take laterally. This is prevented by:
- providing heavy sleepers, that generate friction on the ballast bed
- providing ample ballast all around the sleepers, especially at the sleeper ends in the ballast shoulder
- ensuring that the sleepers are well supported on consolidated ballast to enable the generation of the friction
- stretching the rails if they are installed in cool or cold weather, so that the expansion on the hottest days is less than otherwise
- prohibiting installation in very sharp curves, where the curvature would increase the tendency to buckle
- prohibiting disturbance of the track for maintenance work when hot weather is expected.
If the rail is held so that it cannot expand at all, then there is no limit on the length of rail that can be handled. The expansive force in a one-foot length of rail at a certain temperature is the same as in a 100 mile length of rail. Early cwr was installed in limited lengths only because of technological limitations. However at the end of the cwr section where it abutted ordinary jointed track, that track would be unable to resist the expansive force and the jointed track might be forced to buckle. To prevent that, special expansion switches, sometimes called breathers, were installed. The expansion switches could accommodate a considerable expansive movement -- typically four inches or so -- in the end section of the cwr without passing the movement on to the jointed track.
The cwr is installed and fastened down at an optimum temperature, to ensure that the highest possible expansive force is limited. This temperature is called the stress-free temperature, and in the UK it is . It is in the upper range of ordinary outdoor temperatures, and the actual installation work tends to be done at cooler temperatures. Originally the rails were physically heated to the stress free temperature with propane gas heaters before they were fastened down; they were then rattled with hand bars and then clipped down. Since about 1963 however hydraulic jacks are used to physically stretch the rails while they are supported on temporary rollers. By stretching the rails to the length they would be if they were at the stress-free temperature, then there is no need to heat them; they can just be clipped down before the jacks are released.
The cwr rails are made by welding ordinary rails together. For many years rails could only be made in lengths of up to 60 ft in Britain, and the factory welding process made them into 600, 900 or 1200 ft lengths, depending on the factory. The process used was a flash-butt process in which high electrical currents are used to soften the rail end, and the ends are then forced together by rams. The flash-butt process is very reliable providing that the factory ensured good geometry of the rail ends.
The long rails could be conveyed to site by special train, and unloaded on to the ground (by chaining the end in position and pulling the train put from underneath the rails). The long rails had to be welded together (or to adjacent track) using a site welding process, and after initial experimentation the proprietary Thermit welding process was used. This was an alumino-thermic process in which a powder "portion" was ignited; the aluminium was the fuel and a metallurgically appropriate composition of molten steel descended into the gap between the rail ends, contained in refractory moulds.
The original SmW process was very sensitive to operator skill, and as the welding was usually the final process before returning the track to traffic, improper pressure was sometimes applied resulting in unwanted improper welds. The improved SkV process was less sensitive and over the years weld quality improved.
It is worth mentioning that jointed track has suffered buckles in the past; the fish-plates need to be removed and greased annually (the requirement was relaxed to bi-annually in 1993) and where this was forgotten or where ballast conditions were especially weak, buckling took place. In addition, if rails were allowed to creep, it was always possible that several successive joints closed up, so that the expansion gap was lost, with inevitable results at the onset of hot weather.
See also
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