It's very rare for HSTs to run with only one powercar, and when they do they will nearly always have a barrier vehicle on the rear, since HST trailers have fixed buckeye couplings so a screw coupling only fitted loco would be unable to couple to the rear either to assist in an emergency, or to shunt the stock at the terminus. This did happen on the WR for a while not long after the HSTs were introduced due to a shortage of powercars. There, the barrier vehicles were fitted with generators to provide train supply and reduce the load on the powercar. Whilst not having to drag the dead rear powercar is an advantage from a performance point of view, speed is restricted to 100mph due to the way the HST's brakes work.