Wooden sleepers in superstructures

Rail superstructures over a railway route consist of a rail bed and the rails mounted on them. The sub-structure forms a fixed basis in order to make the superstructure. This balances out the unevenness of the land.

The superstructure, and in particular the rail bed, serves to take and distribute forces which occur from weight, acceleration, sinus runs, speed of the rail vehicles as well as thermal loads caused by weather.

Wooden sleepers are well-suited for all sub-structure and bed types. It can be used on soft as well as field soil, on gravel beds and crushed rock beds.
Wooden sleepers are preferred for use for rail sections where soil settling can be expected (such as mining areas)

All types of fasteners can be used on wooden sleepers, in particular the simpler ones such as spring nails, among others. The simplest and lowest-cost fasteners are DS 18 double shaft spring nails.

The KS superstructure (SKL 12 tension clamps) differ from the K superstructure only with their SKL 12 tension clamps with hook screws and underlying discs instead of clamping plates, hook screws and washer springs. SKL 12s are already pre-mounted in the wooden sleepers. The picture shows the SKL 12 tension clamp on the right of the rail in the pre-mounting position. The left shows the rail in the final assembly position.

Pre-mounting the SKL 12 tension clamps in the factor leads to a significant cost-savings for the KS superstructure for installation or fastening the rails at the construction site.

The K superstructure with ribbed plates, sleeper screws and spring washers as a rail base (if desired, also available with intermediate plastic layers) as well as clamping plates, hook bolts and spring washers, is a good and common superstructure form for fastening rails to a wooden sleeper.

It is advisable to place a plastic intermediate plate (ZW) under the rail footing in order to prevent damage to or abrasion of the rail footing and ribbed plate when the rail rolls off.

Movements also occur when forces are transferred from the rail to the sleeper within the rail fastening as a result of changes in the track grate’s position in the ballast bed under vehicle loads. According to this, this must be structured in such a way that the rails remain permanently anchored on the sleeper with no changes.  This means that the connection between the rails and wooden sleepers must be immovably solid, but the rail’s movements must be accepted by the wooden sleepers. Rail fastening must fulfil the following requirements in order to resist these effects for years, and to keep maintenance costs as low as possible:

The rails must be tensioned against the wooden sleepers. They must neither lift, nor tilt sideways, nor slip under the fasteners due to the effects of temperature forces over the longitudinal direction except for limited elastic movements. The rails must therefore be clamped against the wooden sleepers. The clamping forces of the tension elements (clamping plates with K superstructure and SKL for KS superstructures) must constantly work against the rail footing. This only occurs if a spring part is placed between them. In the K design, this is the spring washer between the clamping plate, hook screw and nut. In the case of KS design, it is a tension clamp with a hook screw, washer, and nut.

Since the KS design’s tension clamp has a significantly improved tensioning and clamping effect compared to the spring washer above the clamping plate in the K design, due to two free spring arms, a permanent force-locking tension is provided for the rail with a long, elastic spring path, whereby no fastening maintenance is required as a rule.

Due to low needs and costs for maintenance, as well as better riding comfort, the Deutsche Bahn switched its structure a few years ago with wooden sleepers on the KS superstructure with SKL 12s pre-mounted in the factory.  In the meantime, other national railways in other countries and other users started to use this system, in some cases other types of fasteners were used.

In summary, one can conclude that rail fasteners are an important design component of the rail for safety and economics of rail operations. They must give good value, be easy to install, provide constant, effective, and long-lasting clamping of the rails to the wooden sleepers, guarantee torsion resistance, require low maintenance costs, and have a long lifespan.