Mobile workshop set
These are tool vans for PW Dept use, constructed from ex-British Telecom demountable van bodies fitted to adapted Hudson underframes. The central wagon 187 provides access to the end doors of the tool vans so the three wagons are kept as a permanently-coupled set.
According to Fred Howes at the time they were built the box bodies are ex-BT demountable van bodies, obtained when BT stopped using them and reverting to normal vans, rather than chaasis/cabs with the demountable bodies. Certainly in the early days before being painted in the grey/yellow livery, the outline of the BT logo was plainly visible from where the vinyls had been removed.