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Material matters

Since railways began designers have realised the importance of using suitable sleepers underneath tracks to transmit loads into the ground below.

Stephenson's Rocket used granite blocks placed under the rail at intervals. But it soon became apparent that the blocks would not support the gauge in pliable soils.

Sawn timber sleepers replaced the stone blocks, and many are still in use on the network today. But the outbreak of the Second World War and the resulting shortage of imported timber meant a replacement had to be found.

Loads from the wheels of rolling stock are transmitted through the rail, sleepers and ballast and into the formation below.

Forces can be up to four times the static wheel loading due to the dynamic effects of the wheel and irregularities of the track. As well as vertical forces, braking and accelerating create transverse and longitudinal forces.

Continuous welded rail, now in use across the network, also leads to increased thermal forces, which have to be restrained.

Sleepers have to hold and maintain the rails to very tight tolerances, vital to ensure the line and level of the track. This has to be done in an aggressive environment, subject to wetting, drying out and dramatic changes in temperature.

To overcome the shortage of timber during the last war, prestressed concrete sleepers were developed, and have been the preferred sleeper type since.

Steel sleepers were introduced to the UK shortly before the war, again to overcome the lack of timber, though steel sleepers had been in use elsewhere since the end of the 19th century. They proved most popular in Africa and the Indian subcontinent due to their resistance to termite attack. Today they are widely used on sections of the networks in North and South America.

Initially, steel sleepers proved popular in the UK, as they were a lot easier to handle than the heavy concrete versions. But throughout the 1950s, as machinery was developed to handle concrete sleepers, steel lost some of its advantages, and it was not until the late 1980s that their level of use began to rise again.

Since privatisation the use of steel sleepers has greatly increased. In 1994 there were approximately 5,000 steel sleepers laid on the UK network, a figure that had risen to nearly 400,000 by 1999.

It was increasing pressure to reduce costs that led track engineers to look again at the merits of steel sleepers, especially for secondary routes where full renewal is not justified.

Andrew McNaughton, now chief engineer at Railtrack, initiated the resurgence of steel sleepers in the UK when he was an engineer in the Manchester region. He is a firm believer in their use, although he admits they are not suitable for all situations. Critics of steel sleepers describe them as a short-sighted, cost-cutting solution, aimed at making short term figures look better.

They claim Railtrack is putting in steel only to replace it with concrete just a few years later when the ballast needs renewing, proving, they insist, that in the long run steel is more expensive.

But McNaughton disagrees.

'Steel sleepers have proved to be a good short term measure, and have proved very cost effective in certain situations, ' he insists.

He says Railtrack has been using steel sleepers successfully on lines where the ballast has not needed replacing, but the existing sleepers have reached the end of their design life.

The existing surface ballast is raked off, or scarified, and the new steel sleepers are vibrated or pushed into the ballast. This avoids the cost of re-ballasting and substantially reduces possession time, reducing disruption to services. Savings can then be used to make additional track upgrades.

McNaughton explains that steel sleepers are only used on sections with a line speed of no more than 90mph, and that when a line requires reballasting concrete sleepers are used.

Manufacturers of concrete sleepers cite the advantages claimed for concrete over steel as reasons why steel use should rise no further. And the Concrete Sleeper Manufacturers Association, (CSMA), has produced a list of downsides to using steel.

It insists that there could be problems with potential electrical insulation caused by the high conductivity of steel. It also stresses the increased maintenance cost of inspecting for these problems.

The CSMA has also pointed to a possible reduction in stability from the use of lighter weight steel sleepers. It insists that the ride quality is inferior to concrete and generates more noise.

One generally accepted problem is that because of the shape of the steel sleepers, mechanical methods of cleaning and renewing ballast cannot be used as the machines cannot get underneath the U shaped section. This is why Railtrack is only using steel sleepers on sections of track where the ballast still has a relatively long life.

But Alan Briggs, President of American Tie & Track Systems, which manufactures steel sleepers and is in the UK working with its agent Pwmm, hits back, saying that the insulation for steel sleepers is exactly the same as for concrete sleepers.

Potentially he says, all sleepers conduct electricity due to surface water or water absorbed into the sleeper. He also believes that the inspection for electrical shorting is simpler on a steel sleeper than on a concrete unit.

Briggs dismisses doubts as to the lateral stability of steel, and claims that in America track stability using steel sleepers has been engineered to be greater than that of concrete, despite the 3:1 weight differential.

Steel sleepers are designed to grab the ballast, he says, whereas ballast below the concrete variant tends to act like marble, behaving like a solid mass. Concrete sleepers, he points out, have no direct interaction with ballast other than weight, while ballast is compacted and held in pockets below the steel sleeper.

Briggs also argues that steel sleepers are not noisy. He claims noise generated by the wheel/ rail interface is dissipated by the ballast.

To support his case further Briggs argued that steel sleepers require less ballast than concrete, less storage space, can be recycled and furthermore dissipate rail heat into the ballast which helps prevent track buckle.

This debate as to the merits of the two sleeper types will continue and is probably best summed up by agreeing that each has different qualities.

But MacNaughton warns that the opposing parties might be better occupied worrying about another development in track technology - slabtrack. This system does away with both ballast and sleepers, using a solid concrete slab to support the rails.

Slabtrack is becoming more popular across the world, and many experts believe it could be the way forward. ST

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