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. Finishing construction Now that the first track is almost complete, work begins on the adjacent track. This time, however, it is not necessary to lay a temporary track. Trains running on the first track bring the sleepers, and then the rail, which is unloaded directly onto the sleepers by dispensing arms that swing out to the proper alignment. The Nabla fasteners are secured, and the ballast is stuffed under the track as before. The two tracks are now essentially complete, but the work on the line is not finished. The catenary masts need to be erected, and the wire strung on them. Once the catenary is complete, the track is given final alignment adjustments down to millimeter tolerances. The ballast is then blown to remove smaller gravel fragments and dust, which might be kicked up by trains. This step is especially important on high-speed tracks, since the blast of a passing train is strong. Finally, TGV trains are tested on the line at gradually increasing speeds. The track is qualified at speeds slightly higher than will be used in everyday operations (typically 350 km/h, or 210 mph), before being opened to commercial service. References Rail infrastructure in France Maintenance of way equipment High-speed rail in France Rail technologies Permanent way