Full brakes, and other non-passenger coaching stock like GUV's, carried quite a variety of traffic. Invent a suitable traffic for your location generating quantities of parcels, and that may be justification enough. National newspaper distribution was still largely on the rail in the 1960's, and that generated regular outward loaded and empty return flows, and could lead to such a vehicle being added to the first train of the day down a branch, returning later. Pigeon racing saw such vans at various locations both collecting and releasing the competitors. Having a strong liking for such vehicles, I use any excuse to run as many as possible.