One of two flat wagons, FB. 578 and FB. 579, which were converted in 1949 as brake vans for use with the experimental 400 h.p. tube dimension diesel-electric locomotive No. 120 of 1941, which was then working ballast trains.  They were fitted with a steel and glass "sentry box" for the guard, and with Westinghouse air brakes, and hand brakes.

When after trials it was decided that D.E.L. 120 was not a success, they were put to use as stores vans, not being heavy enough for use as brake vans with steam hauled ballast trains.

To save having to use both a brake van and a match wagon when the "long-welded rail train" was to be hauled by a steam loco, in 1961 they were modified to their present form.  This was achieved by increasing the weight to 18 tons 6 cwt, and building full width cabs.  The handbrake rigging was modified to give the necessary increase in braking force, and the Westinghouse equipment removed.  The main and train line pipes and hose couplings were retained to allow working in trains hauled by air braked battery locomotives.  Detachable side boards were fitted to the flat sections of the vehicles, allowing up to 2 tons of material to be carried.

With the end of steam working on London Transport they were used as match wagons for hauling scrap 1938 Tube Stock out over British Rail tracks.  As Tube Stock have no couplings at normal height British Railways locomotives and Wagons were unable to couple directly to the Tube Stock, hence the use of these wagons.  When this service finished these vehicles were stored at Ruislip until 1982.  FB. 578 arrived at Quainton 28th June 1984, while FB. 579 was cut up by LT for scrap on 16 December 1983.

FB. 578 is stabled with a flat wagon, F. 329, which is similar to the one modified to form FB. 578.