Abstract: A carriage (11) has a carriage body (12) to extend across the top face of a track. A fixed bearing arrangement (14) includes a bearing (15a, 15b) to engage a first side of the track and having a rotational axis located in a fixed position on the carriage body. A self-adjusting bearing arrangement (19) includes a movable bearing (20) to engage a second side of the track and mounted on a pivot arm (21) having a pivotal connection (24, 25) with the carriage body. A bias spring is arranged to act on the pivot arm such as to maintain contact between the movable bearing and the second side of the track. The movable bearing (20) is not driven. The movable bearing (20) and the pivotal connection (24, 25) of the pivot arm (21) are both located at a first end of the pivot arm. The bias spring is spaced from the pivotal connection (24, 25) in a first direction.