Steel sleepers are being used in ever greater numbers as a replacement for concrete.
This week we ask: Is Railtrack right to continue increasing the number of steel sleepers on the network?
Pre-stressed concrete sleepers are designed for a 30 year life. When I was divisional civil engineer for British Rail in Manchester in the late 1980s, many sites had poor ballast conditions with failed concrete sleepers. They had failed either through breaking their backs on centre bound ballast or by the soffits being worn away by attrition, exposing the prestressing wires. While the track geometry was generally being maintained, sleepers were failing in large numbers after 10 or 15 years.
Steel sleepers have a demonstrably longer life. In Germany there are steel sleepers in the track which date back to the early 1940s. When steel sleepers were installed in north west England in the 1980s, our intention was to renew quickly and at low cost to eliminate speed restrictions. The use of light section steel sleepers with their high tolerance of sub-standard ballast conditions was important.
Concrete sleepers on mixed traffic high speed lines are subject to increased attrition due to voiding and ballast degradation that reduces their life in the track. Steel sleepers rely on the interaction between the ballast trapped in their hollow undersides and the ballast bed. Although they are currently restricted to 90 mph, I believe that, as railway engineers, we should consider broader section sleepers with more heavily spaded ends as a potential alternative to concrete, even on high speed heavy axle routes like the West Coast Main Line.
Continental experience has shown that rusting is not a factor in the life of steel sleepers. The suggestion that track circuits are dropped more often with steel sleepers is also a myth when the site is well controlled. Several UK contractors are experts in steel sleeper installation and over a third of a million are installed each year.
After 175 years of railways in the UK, permanent way engineers should continue to apply their knowledge of engineering principles towards continuous improvement, and the development of steel sleepers for use at higher speeds is worthy of everyone's attention.
Ballasted track using prestressed concrete has been used successfully in many countries for many years - here in the UK for more than 55 years. With today's modern rail fastenings, concrete sleepers are part of the permanent way and with adequate track maintenance will have a life over 50 years. This type of track provides good ride quality, good retention of line, level and gauge and a high resistance to track buckling. It can be laid and maintained with the new generation of high performance machines and in particular the stoneblower.
Stoneblowers only have to be used half as frequently as tampers whose operation, unlike stoneblowers, reduces the ballast life. Track can easily be realigned if required or reinstated when damage occurs, for example through derailments, embankment slips or flooding.
Steel sleepers have only been used in a few countries and even then to nothing like the same extent as concrete. In many cases steel sleepers have been abandoned. In the UK their use is normally restricted to those secondary lines where re-ballasting can be postponed for at least 10 years and where, unlike concrete, they can be laid on the existing compacted ballast. The saving in ballast costs enables sleeper renewals with a reduction in initial cost.
This is sound sense on those lines, which would otherwise not be kept open. However, steel sleepered track is more expensive to maintain and stoneblowers cannot be used. Further, it is generally considered unsuitable with new ballast because of the difficulty in establishing and then maintaining good line and level. It is also questionable whether there is enough knowledge about possible safety matters including electrical insulation, additional inspection procedures and the reduction in lateral track stability because of their reduced weight.
It has been suggested by Railtrack that after expiry of the ballast life steel sleepers will be replaced by concrete. The 'whole life' costing model by Booz Allen & Hamilton commissioned by the CSMA, demonstrates that concrete sleepers would have been the best initial choice.
Loads from the wheels of rolling stock are transmitted through the rail, sleepers and ballast and into the ground underneath the rail.
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.
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.
Prestressed concrete sleepers were developed to overcome the shortage of timber during the last war and have been the preferred sleeper type since.