motorized lorry 34 comprises rail wheels 51 for travelling on the existing track 31 and a retractable off track travel mechanism the mechanism 50 is a caterpillar mechanism for example which is lowered by arms 52 to below the level of the rail wheels 51 other off track travel mechanisms can be envisaged such as a wheel mechanism for example equipped with all terrain tyres therefore in retracted position as illustrated in fig1 and 18 the off track travel mechanism 50 is lifted to place the rail wheels 51 in contact with the rails 53 in deployed position as illustrated in fig1 and 20 the off track travel mechanism 50 is lowered to rest upon the platform 30 so as to lift the motorized lorry 34 until its rail wheels 51 are lifted off the rails 53 the motorization of the lorry 34 is of hydraulic remote controllable type for example the purpose of the beam 44 is to maintain the panel 32 flat in the absence of a beam the panel which may be 36 meters in length or more for example even only 24 meters or less will tend to bend under its own weight to which must be added the weight of the gantries 33 when it is not laid on the ground on.