compacted by rollers, provides an adequate surface for vehicles with tyres. TGV tracklaying then proceeds. The tracklaying process is not particularly specialized to high-speed lines; the same general technique is applicable to any track that uses continuous welded rail. The steps outlined below are used around the world in modern tracklaying. TGV track, however, conforms to stringent requirements on materials, dimensions and tolerances. Laying the track To begin laying track, a gantry crane that rides on rubber tires is used to lay down panels of prefabricated track. These are laid roughly in the location where one of the tracks will be built (all LGVs have two tracks). Each panel is 18 metres (60 feet) long, and rests on wooden sleepers. No ballast is used at this stage, since the panel track is temporary. Once the panel track is laid, a work train (pulled by diesel locomotives) can bring in the sections of continuous welded rail that will be used for the permanent way of this first track. The rail comes from the factory in lengths varying from 200 m (660 ft) to 400 m (1310 ft). Such long pieces of rail are just laid across several flatcars; they are very flexible, so this does not pose a problem. A special crane unloads the rail sections and places them on each side of the temporary track, approximately 3.5 m (12 ft) apart. This operation is usually carried out at night, for thermal reasons. The rail itself is standard UIC section, 60 kg/m (40 lb/ft), with a tensile strength of 800 newtons per square millimetre or megapascals (116,000 psi). For the next step, a gantry crane is used again. This time, however, the crane rides on the two rails that were just laid alongside the temporary track. A train of flatcars, half loaded with LGV sleepers, arrives at the site. It is pushed by a special diesel locomotive, which is low enough to fit underneath the gantry cranes. The cranes remove the panels of temporary track, and stack them onto the empty half of the sleeper train. Next, they pick up sets of 30 LGV sleepers, pre-arranged with the proper spacing (60 cm, or 24 in), using a special fixture. The sleepers are laid on the gravel bed where the panel track was. The sleeper train leaves the worksite loaded with sections of panel track. The sleepers, sometimes known as bi-bloc sleepers,