Definition of "single-sided"

Unlike the "West Coast Postal" of the L.M.R., the Western Region TPO train is not turned at each end of its journey, and as the coaches with pick-up/set-down apparatus are single-sided, one coach serves the mail exchange points on the down journey, a second coach marshalled the other way round serving the up exchange points.

1959 December, “New TPOs for the Western Region”, in Trains Illustrated, page 581