Abstract: A single overhead pneumatic tube system employs a large rectangular tube, a pair of overhead mounted blowers and a large rectangular captive carrier having a pair of oppositely disposed sliding covers which are locked in position when the carrier is passing from one terminal to the other. The locking means for each cover includes a pair of diagonally offset bolts having heads exposed through apertures in the cover. The locking bolts of the two covers are interconnected so that withdrawal of the bolts of one cover by pressure on their bolt heads also withdraws the bolts of the other cover and so unlocks both covers at the same time.