A track frame or undercarriage (12) is made to support a rubber track (10) on suitable guide rollers (13,18,19). A tensioning roller (18) is provided as part of the guide rollers (13,18,19) on the interior of the track (10) and is mounted on a pivoting bracket (24). The bracket (24) moves the tensioning roller (18) to a working position for creating tension in the track (10). An actuator (22) provides a known force on the bracket (24) to move the tensioning roller (18) to its working position, using a controlled pressure applied to the actuator (22). Drive sprockets (19) for the track (10) are mounted onto a drive case (20) that is mounted to the track frame (12) through pivoting linkages (40,42) such that the drive case (20) and the drive sprocket (19) can be held in a working position and retracted away from the track (10). The position of the tensioning roller (18) is maintained within set limits of retractions during use, by providing a stop (32,30) to limit the amount that the tensioning roller (18) can retract.