pair of lateral wings 9 by means of which it is attached to a respective track belt 5 a 5 b for movement with the track belt the track 5 is mounted and driven so that the traction elements 7 describe a path in the form of an endless loop the path includes a path segment 11 in which the path is essentially straight and aligned with the longitudinal axis 13 of the member pipe or cable or isu etc 15 each traction element 7 is essentially similar and consists of a base unit 17 on which are mounted pads or cushions 19 which define gripping surfaces 21 which contact the member in use the two lateral wings 9 depend from the respective sides of the base unit 17 and may be formed integrally therewith at either side of the base unit 17 are mounted arms 23 a and 23 b on each arm is mounted a pad or cushion 25 which is generally similar to the pads or cushions 19 and which defines a further gripping surface 27 a 27 b the arms 23 are mounted on the base unit so that they can rotate with respect to the base unit 17 about axes 29 a and 29 b the axes.