Sleepers and bearers or timbers (for points and crossings) need to fulfil the following basic functions:
•Spread wheel loads to ballast.
•Hold rails to gauge and inclination.
•Transmit lateral and longitudinal forces.
•Insulate rails electrically.
•Provide a base for rail seats and fastenings.
Sleepers are also often called upon to fulfil other secondary but important functions which should not be overlooked. These include:
•Supporting wheels and/or jacks direct (in a derailment situation).
•Acting as transverse beams when sitting on temporary ?waybeams'.
•Supporting signal engineering and other safety related equipment such as trip cocks and point motors.
•Supporting conductor rails, electrical bonds and feeder cables.
•Reducing noise and vibration on non-ballasted bridge decks.
The traditional timber sleeper was accepted by most railways as standard up to about the middle of the twentieth century, although its durability limitations were recognised.
Even today there are still many railways using timber sleepers, where
the advantages of good resilience, ease of handling, adaptability to nonstandard situations or electrical insulation are very important.
Timber sleepers and bearers for surface railways are usually made of softwood, either imported Douglas Fir or homegrown Scots Pine. The standard dimensions for softwood sleepers used in the UK are 254mm wide by 127mm thick in cross-section by 2600mm long.
All softwoods used in sleepers and bearers must be thoroughly seasoned and then impregnated under pressure with a suitable preservative before use. Traditionally this preservative has been hot creosote but other materials have been used successfully in recent years which may have less associated health hazards.
All lines in deep tube tunnels or in locations where fire could be a risk are provided with sleepers and pit-blocks made from imported untreated hardwood such as Jarrah.
Jarrah timbers used on the surface for points and crossing work which is not protected from the weather can last up to 35 years. In the protected environment of dry tube tunnels, Jarrah sleepers on the London Underground have been known to last in excess of fifty years before needing renewal.
The author has in his possession a handsome polished jarrah pen and pencil box which bears the following interesting inscription under the lid:
?This box is made from jarrah sleepers withdrawn from the London Tube railways after 54 years continuous service. It is estimated that during this time 500 million passengers travelled over the sleepers.'
Such comment speaks for itself. Hardwood sleepers eventually usually need replacing after this long period not because the general condition of the timber has deteriorated but because it is by then not possible to get a sound fixing for chair screws.
Softwood treated sleepers on the surface can be expected to last between 15 and 25 years depending on location and traffic use. Renewal is usually
required because bad splitting and/or rot has occurred.
Prestressed Concrete Sleepers (Monobloc)
As a substitute to softwood some experimental work was carried out during the late 1930's on concrete sleepers. Origin was used but not found very satisfactory for a number of reasons. At that stage, concrete simply replaced timber, bullhead rails and cast iron chairs being used as in other conventional track.
After the Second World War prestressed concrete was developed and used extensively on new structures. The great advantage of prestressed concrete was that concrete is kept under compression under all conditions of flexure, both under load and after. This means that tension cracks do not occur which can allow the ingress of moisture and corrosion of embedded steel.
Development of prestressed sleepers took place about the same time as development of flat bottomed rail and direct fastenings.
At the time of writing the standard sleeper for main line railways in the UK is the F27(AS or BS) prestressed concrete sleeper manufactured by the pretensioned method. Variations of this standard sleeper are available with extra holes for supporting conductor rail insulators and with shallower depth where these conditions apply. In this method the prestressing tendons are tensioned prior to the concrete being placed and are only released once the concrete has reached sufficient compressive strength to resist the induced forces thus applied. This method is also sometimes referred to as the ?Long Line' system, as sleepers are cast five sleepers or more.
Some counties outside the UK adopt the post-tensioning method where tendons are placed in debonding sheaths and the stress is applied after the concrete has hardened by application of tensile force to the tendons by jacking and final anchoring. This method is slower but less capital intensive and lends itself to small-scale production and situations where demand is less.
Standard prestressed concrete sleepers used in the UK are normally 2515mm long by 264mm wide. The depth varies from 203mm at the rail seat to 165mm at the centre line giving a total weight of 285 kg. The prestress is provided by six No. 9.3mm strands for standard use increased to eight and strands for heavy duty. These sleepers are capable of sustaining an equivalent dynamic load of 24 tonnes at each rail seat. Allowing for impact, lurching, wheel flats, poor rail joints and etc this is equivalent to the effects of the passage of a static 25 tonnes axle.
Metros and light rail systems have extensively also adopted prestressed concrete sleepers. Where maximum axle loads are less than for main line as shown above, the sleeper dimensions may be reduced accordingly. However, great care must be taken in the design to ensure that adequate allowance is made for dynamic effects, particularly fo bending moments.
The main disadvantage of the concrete sleeper over its timber predecessor is that of weight. Timber sleepers were often manhandled into their final position and replacement of single def it is sometimes known) was also done by hand. With concrete sleepers some form of mechanisation is required for these operations.
Twin Block Sleepers
The twin block sleeper consists of two reinforced concrete blocks joined together with a steel tie bar cast into the blocks. This type of sleeper is used extensively in Europe, particularly in France, but not in the UK. The standard sleeper weighs 230 kg which is less than the monobloc equivalent. However handling and placing can be difficult due to the tendency to twist when lifted. Twinblock sleepers can be provided with resili can be incorporated into non-ballasted slab track or monolithic embedment in road surfaces for light rail street running.
Steel sleepers have been hardly ever used in the UK, largely because of cost and fear of corrosion in our variable weather conditions. However, there are countries throughout the world where these sleepers are used successfully, particularly where trains run at moderate speeds only. Reference should be made to BS 500. Most steel sleepers are inverted troughs which are either rolled to that section or rolled flat and then hot pressed to the trough shape. Being only 68 kg in weight, these sleepers are easy to handle but the inverted trough makes them difficult to satisfactorily pack with ballast. They have been shown to be completely satisfactory however in sidings and depots. Electrical insulation is necessary at fastenings if track circuits are being used for train detection and this is not always a simple or effective matter.
In some climates the normal coating of millscale and rust is sufficient to protect against significant loss of section by corrosion. Steel sleepers can however be given protection by dipping in bitumen or oil during the production process.