Steel sleepers
Steel sleepers are specified in accordance with the DIN 5904 standard. Their design allows them to be used even where height is limited. The characteristic trough shape (see picture below, right), which has become established despite country-specific differences in geometric design, ensures high positional stability in the ballast bed when correctly tamped. Advances in fastening technology, such as the factory welding of ribbed plates, have significantly minimised the initial challenges during installation.
The steel trough profiles are rolled profiles that are sawn to the appropriate lengths in the sleeper factory. The two ends are then mechanically bent to increase the resistance to lateral displacement. In the same work step, depending on the rail fastening system and customer requirements, the sleepers can be bent under the subsequent rail supports so that the desired rail inclination (1:20 - 1:40) is present directly in the profile if no inclined ribbed plate is required.
© Patrick Braess
CO2 emissions during the rolling of the raw profiles have been significantly reduced by continuous optimisation of the steelworks. For example, rolling profiles at the Stahlwerke Thüringen (stahlwerk-thueringen.de), one of the main suppliers of raw profiles, now only produces 327 kg of CO2 per tonne of steel. This puts the steelworks in a leading position worldwide in terms of sustainability. Steel sleeper sections there are made from 100% old steel, which is melted down using the electric arc process. The blast furnace is powered by renewable energy, meaning that the steel is classified as green steel.
The advantages of steel sleepers are manifold: Thanks to their shape and embedding in the ballast, they offer excellent positional stability. New steel sleepers have been manufactured with 22 cm long caps (the bent wings) for around 20 years. This increases the resistance to lateral displacement and guarantees high positional stability even in tight curves.
Their low weight makes handling considerably easier and the robustness of the steel material promises a long service life. The service life of modern steel sleepers is guaranteed by the manufacturers to be up to 75 years, but there are also much longer service lives in the railway network.
© Patrick Braess
In addition, recycling is possible through reconditioning, which further increases the sustainability of this material. At the end of their service life, steel sleepers can be 100% recycled. Due to their purity, they are highly valued as a raw material in steelworks.
Special steel sleeper profiles are also sometimes used at bridge crossings or to accommodate the drive technology of points.
However, there are also disadvantages to consider: The high conductivity of steel can lead to problems with the insulation of the signalling current. Insufficient embedding in the ballast can impair track stability. In addition, derailments can lead to severe deformations caused by the pressed-in wheel flange, which can cause the track to narrow.
© Schwihag AG
Nowadays, modern steel sleepers in Switzerland are supplied with the ST(i) 14 rail fastening. The standard SKL 14 tension clamp, which is also used for B70 and (in Switzerland) B91 concrete sleepers, is partially plastic-coated so that stray currents no longer affect the track circuit. Any necessary track widening in the curve or corrections to the track gauge can be made simply by replacing the angle guide plates.
To ensure that the hook bolts can be replaced at any time, there is a welded U-profile underneath in the sleeper trough to prevent jamming with the ballast.
Y-shaped steel sleepers
Over the years, another form of steel sleeper has developed from the trough sleeper: the Y-sleeper. It takes its name from its characteristic shape. Two curved I-shaped steel girders (the I stands for the characteristic cross-sectional shape of the girder) connect six rail support points (three double supports). The two steel girders are welded together via steel flanges at the top and bottom.
Thanks to this special design, tracks with Y-sleepers have a very high horizontal positional stability despite their comparatively low weight of approx. 143 kg per sleeper. This is due to the very high frame rigidity of the Y-sleeper track grid. With a length of 2.30 m, the Y-sleeper also has smaller dimensions, which makes it particularly interesting for use with smaller ballast widths. The track position must be corrected using special tamping machines, which are able to adjust the tamping unit to the special sleeper shape. Despite a number of advantages, the Y-sleeper has not become established due to the higher unit price and the special maintenance requirements.
In friendly cooperation with Tensol Rail SA
You can find suitable specialist literature to the topic here:
The Basic Principles of Mechanised Track Maintenance
This book is dedicated to the many people involved in the day to day planning and performance of track maintenance activities. Providing a practical approach to everyday challenges in mechanised track maintenance, it is not just intended as a theoretical approach to the track system.
Railways aim at transporting people and freight safely, rapidly, regularly, comfortably and on time from one place to another. This book is directed to track infrastructure departments contributing to the above objective by ensuring the track infrastructure’s reliability, availability, maintainability and safety – denoted by the acronym RAMS. Regular, effective and affordable track maintenance enable RAMS to be achieved.
- [1] Hansmann F., Nemetz W.: Der Gleislage auf der Spur. TrackoMedia, 2019.