Since 2005 short workings on London’s bus route number 15 (the portion between Tower Hill and Trafalgar Square) are operated as a “heritage” route. Te service utilises five "Routemasters". The four prototype Routemasters began service in 1956 and the production vehicles were introduced from 1959. The maintenance regime operated by London Transport saw vehicles undergo a comprehensive overhaul at Aldenham works every three or four years and this involved stripping the entire vehicle down. The same parts were not reassembled into the same bus so it is impossible to say how old each individual vehicle is. However, production ceased in 1968 so those still running in London are between 50 and 60 years old. That said, all now have low-emission engines completely different from the original gas-guzzling 9.6 litre AEC units. There were originally two “Heritage “routes, though a similar arrangement on Route 9 between Aldwych and the Royal Albert Hall ceased in 2014.