That's really a unique case, the first southbound and last northbound Central Wales trains were really the postal service, which had for long run vans through from York to Swansea but was curtailed at Shrewsbury. It's unreasonable to have built the 100 WR cross-country units with this wasted space just to suit this one operation, which only converted from loco to dmu when the units were about 10 years old.I didn't say every service was carrying vast amounts of mail and parcels all day, every day, all year, did I? The dmus needed to be able to handle whatever loads were piled into them.
The Scottish Region handled this better, and stripped out all the seats from a few former WR single units, turning them into parcels cars, which multipled with the normal dmu provided. I once escorted some USA visitors from Inverness to Edinburgh, changing at Stirling, where a 3-car Met Cam was waiting in the bay, where I ushered them into the frontmost seats for an impressive view on the last lap. Five minutes before departure one of these single parcels cars crept out of a siding and coupled up right in front of us. End of view.