Superstructure (railway)

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Gravel superstructure with wooden sleepers

The superstructure or track structure of a railway line consists of the track bed and the tracks mounted on it . The substructure forms a solid template for the construction of the superstructure by compensating for unevenness in the terrain .

The superstructure, and in particular the track bed, serves to absorb and distribute the forces that arise from mass , acceleration , sinusoidal motion , the speed of the rail vehicles and thermal loads from the weather .

Of the functionality of the superstructure and substructure, among others, the maximum speed of a track and the permissible hang axle load from the vehicles, they will be at defects in the superstructure for safety speed restrictions established.

In Germany, the Federal Railway Authority is responsible for compliance with the legal requirements , and DB Netz has been obliged to maintain and operate the rail network .

construction

Slab track on the Nuremberg – Ingolstadt SFS ; System Bögl

The superstructure initially contains a track bed, which is usually made of gravel . On this "ballast superstructure" sleepers are placed transversely to the direction of travel , on which the rails are attached in the direction of travel . The sleepers, which are usually made of concrete , but often made of wood or steel , keep the rails at the intended distance, the gauge , from one another and, in the case of continuously welded rails, must also transfer the thermally induced linear expansion forces into the substructure.

Rails are held on the sleepers by fasteners. In the simplest case, these are nails driven into the wooden sleepers, one flank of the T-head protruding over an edge of the rail foot. Currently i. d. R. Systems of threaded bolts, nuts and clamps used. Fastening methods commonly used in Germany are superstructure W on concrete sleepers and a variant of superstructure K on wooden sleepers .

The ballast bed is flexible and helps to transfer both static and dynamic loads evenly to the substructure. Other advantages of a ballast bed are relatively good noise insulation and the ability to absorb vibrations caused by the sinusoidal movement of trains. A ballast bed can be maintained with simple means. Furthermore, a gravel bed drains the usual amounts of precipitation in Europe into the ground. The disadvantage is that during high-speed journeys, gravel can be torn up from the track bed by the draft of the moving train (“ gravel flight ”) and poses a risk for people and animals in the vicinity as well as for the technology.

In addition to sub-swelling by means of cross sleepers, longitudinal sleepers are known. These are installed lengthways under both rails. Sleeper screws with clamping plates, casting compound or clamping jaws are used for fixing. The application assumes that the forces to be transmitted can be transferred to the subsurface via a solid base.

The track bed can also be manufactured as a slab track , in which the rails are mounted directly on a fixed superstructure made of concrete or asphalt . The concrete superstructure is used primarily on modern high-speed and high-speed lines, while asphalt-based superstructures are mainly used on trams that have to share their routes with motor vehicle traffic. A solid track bed requires sound insulation in the form of insulating mats as an additional superstructure element .

maintenance

Ballast cleaning machine used by Deutsche Bahn AG to replace the ballast in the track bed. You can see the front part, which lifts the track and collects the ballast stones.
Rear part that dumps the ballast in the right size.

The superstructure needs regular maintenance work in order to be able to fulfill its tasks. Inadequate maintenance reduces the load-bearing capacity of the track bed, and the result is the establishment of speed limits.

swell

Consequences for the superstructure after a freight train has traveled with a derailed wagon at Hofheim (Ried) station on the Darmstadt – Worms railway line : Destroyed sleepers can ensure that the affected area is declared impassable.

Even with the best impregnation, wooden sleepers begin to rot after a while and then no longer provide a stable base for the rails. They therefore rarely have a lay-time of more than 25 years. In special environments, for example in damp tunnels , the length of stay can be significantly shorter. If the fasteners can no longer find a hold in rotten sleepers, the track can widen, which can even lead to derailments .

Even steel and concrete sleepers aging and must be replaced in time therefore.

More recent developments, on the one hand the Y-steel sleeper for the normal superstructure, on the other hand the steel bridge sleeper (SBS), have brought significant improvements in terms of positional stability and length of stay. Both systems are designed in such a way that a long service life and a short processing period can be expected.

Gravel bed

The ballast bed also needs maintenance because

  • A perfect drainage of rainwater into the substructure must always be guaranteed,
  • the ballast bed must remain elastic through sufficient internal cavities and
  • the ballast stones have to claw their way through sharp edges to form a stable base.

Over time, plants and the humus they hold clog the gaps. Rising mud from the substructure can also be pumped up through the ballast bed due to movement by rail vehicles and lead to the same problem. Finally, the ballast loses its sharp edges due to its movement and then migrates outwards under the sleepers.

The following measures are therefore taken for maintenance:

  • The easiest way to apply herbicides is to use weeds to prevent vegetation from coming up .
  • Emigrated ballast is supplemented by new ballast, which is brought in with a tamping machine during a tamping pass (the ballast is, so to speak, “tamped” under the sleepers; in fact, the process is vibrating). In technical jargon , this plug grampen and the machine to track tamping machine , Gramper or Grampgerät.
  • If the ballast bed is either heavily contaminated, i.e. hardly contains any voids, or is heavily worn, a complete replacement of the ballast bed is necessary.

The time it takes for the ballast bed to be worked through depends heavily on the load and the desired positional stability. As a rule, attempts are made to clean the existing ballast with ballast cleaning machines and to bring it back in, in order to keep the need for new ballast as low as possible.

rails

The rail itself also has to be replaced regularly, because certain wear and tear can no longer be remedied by grinding with a grinding trolley. For example, the outer rail stays in a spiral tunnel on the Gotthard Railway for around three years. Grinding can only be carried out a few times because the rail head must not fall below certain minimum dimensions. The constant alternating load on the rail can also cause microcracks which - if they are not recognized in good time - lead to a rail breakage. Nowadays, the main routes are regularly scanned with ultrasonic test vehicles for the early detection of damage .

The previously regularly pertaining linesmen are now mostly replaced by vehicles that necessary controls economic conduct.

See also

literature

  • Ulf Gerber: Design of the railway superstructure . In: L. Fendrich, W. Fengler (Hrsg.): Handbuch Eisenbahninfrastructure . Springer, 2013, ISBN 978-3-642-30020-2 , pp. 39-64 ( full text ).

Web links

Commons : Oberbau  - collection of images, videos and audio files

Individual evidence

  1. ↑ Grammers stuff gravel under the sleepers