TGV Track Construction - Adding Ballast

Adding Ballast

The next step consists of stuffing a deep bed of ballast underneath the new track. The ballast arrives in a train of hopper cars pulled by diesel locomotives. Handling this train is challenging, since the ballast must be spread evenly. If the train stops, ballast can pile over the rails and derail it.

A first layer of ballast is dumped directly onto the track, and a tamping-lining-levelling machine, riding on the rails, forces the stones underneath the sleepers. Each pass of this machine can raise the level of the track by 8 cm (3 in), so several passes of ballasting and of the machine are needed to build up a layer of ballast at least 32 cm (1 ft) thick under the sleepers. The ballast is also piled on each side of the track for lateral stability. The machine performs the initial alignment of the track. Next, a ballast regulator distributes the ballast evenly. Finally, a dynamic vibrator machine shakes the track to perform the final tamping, effectively simulating the passing of 2,500 axles.