The British Rail TC (Trailer Control) multiple units were unpowered fixed formations of 3 or 4 carriages with a driving position at each end of the set, converted by BR's Holgate Road carriage 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. Operation The units were primarily employed on services between London Waterloo and Weymouth. One or two 4TC units would be propelled from London to Bournemouth by a 4REP unit, controlled from the leading cab. At Bournemouth, one or both 4TCs would continue over the then unelectrified line to Weymouth, hauled by a Class 33/1 diesel locomotive, leaving the 4REP at the London end of Bournemouth station. Up trains from Weymouth would follow the same procedure in reverse: the Class 33/1 locomotive would propel its train to Bournemouth (controlled from the 4TC leading cab). There the 4TC/s would couple to the waiting 4REP which would then power the whole formation to London, leaving the Class 33/1 locomotive at Bournemouth to await the next such service. 4TC units were occasionally seen on other duties such as Waterloo to Salisbury, the Clapham Junction to Kensington Olympia shuttle before that line was electrified and surplus units were also used on Sundays to operate Portsmouth Harbour to Reading direct services during the late 1970s & early 1980s. They were pressed into general service as needed, often deputising for electric trains when the power was switched off and occasionally on inter-regional Southern Region services. They ventured further afield on special duties or rail-tours, including Birmingham, Cardiff, Barnstaple/Meeth & Meldon Quarry. Until the closure of the Swanage branch, a small number of through 4TC workings to/from Waterloo were operated, also using class 33/1 locomotives in similar style to the Weymouth trains. The spare driving trailer second open (DTSO) number 76331 was equipped with an 'updated' interior - the seats were replaced with Inter-City style individual bucket seats (as used in the contemporary Mk2A/B/C carriages)