High Quality Content by WIKIPEDIA articles! The British Rail TC (Trailer Control) multiple units were un-powered fixed formations of 3 or 4 carriages with a driving position at each end of the set, converted by BR at York Works from locomotive-hauled Mark 1 carriages in 1966-1967 and 1974. The units built on experience gained from the prototype 6TC unit. In time the 3 car units (3TC, numbered in the series 3xx) were reformed into four car units (4TC numbered in the series 4xx) to match the rest of the fleet and later classified as Class 442. This was later changed to Class 491, under which they spent the majority of their working lives. Shortly before withdrawal they were reclassified Class 438 and the units were renumbered to 8001-8034.