Barrier coaches are used when unit train (DMU, EMU or HST) coaches needed to be moved to works for overhauls or maintenance tasks beyond the capabilities of the depots. Frequently these coaches have couplers intended only for use within a set or unit, so cannot easily be coupled to other rolling stock. Using barrier coaches fitted with the unit train coupler and conventional side buffers either side the group of coaches can then be coupled to a locomotive.
Occasionally an HST power car would be sent from one depot to another under it's own power, in which case a barrier coach would often be coupled to be rear/inner end, making a quite unusual train.