carry out the process according to the invention will now be described with more specific reference to fig2 and 3 the vehicle preferably comprises two pairs of crawler chassis 201 202 203 204 the chassis of a pair are spaced laterally to allow installation of supports 205 which in fact are rails on which the bogie 101 rolls when it is loaded onto or unloaded from the vehicle 2 the crawler chassis of each pair are connected rigidly together and each pair is connected to the other likewise rigidly or in some cases by means making it possible to absorb the irregularities of the platform in the example illustrated in fig2 and 3 the crawler chassis 201 to 204 are connected by means of two rectangular beams 206 207 fig2 which is a side view of the vehicle 2 without the chassis 204 shows diagrammatically the crawler chassis 203 comprising a chain or tread 208 rollers 209 two gearwheels 210 ensuring the drive the front wheel of the vehicle in the direction of movement during the laying of the new track is not shown in order to avoid overloading the drawing the rails 205 on which the bogie rests and which bear on the sleepers rectangular beams 206 207 connecting the four crawler chassis and a device making it possible.