and accommodation units for worksite personnel are some examples. A small number of railtour companies have waivers to operate Mark 1 carriage stock on railtour duties. The conditions usually involve the fitting of central door locking and having either a non-passenger or non-Mark 1 carriage at the ends of the train. This is often achieved by running with a locomotive at each end of the train, instead of just at the front, which also reduces the need to run-round at minor terminus stations, many of which lost their run-round facilities when locomotive hauled trains of Mark 1 or Mark 2 coaches gave way to multiple unit operation on service trains. The final withdrawal of Mark 1s from daily mainline use in 2005, some 54 years since the design entered service. South West Trains was granted a derogation to continue to operating two three-car 421s on the Lymington branch line until 2010. Due to the lack of central door locking and Mark 1 stock not meeting the latest rolling stock safety expectations, various rules now govern their usage. In particular vehicles lacking central door locking require a steward to be present in each vehicle. Train companies are also recommended to run Mark 1 stock with more robust non-Mark 1 stock at either end to act as a barrier in the case of collisions. Mark 1s continue to be used on special charter trains with charter operators including North Yorkshire Moors Railway, Riviera Trains, Vintage Trains and West Coast Railways. The Office of Rail & Road has granted these operators an exemption to the Railway Safety Regulations 1999 to continue to operate Mark 1s on the main line until 2023. Preservation Restored Mark 1 carriages have entered service on many of the UK's heritage railways. Their ready availability has avoided the need for the railways to rely on the limited quantity of surviving pre-BR carriage stock – most examples of which had been scrapped before the railway preservation movement had properly started. The long service life of Mark 1 carriages means that a heritage railway can (potentially) recreate an authentic period train of the 1950s, 1960s, 1970s, 1980s or early 1990s, when painted in a suitable livery. Sleeping cars Several Mark 1 sleeping cars were bought by preservation societies for use as volunteer accommodation. When British Rail began to run down its overnight sleeper trains, many heritage railways replaced them with 'young'