by private sector companies under contract to London Buses. With the introduction of the London congestion charge in central London and because at peak times the Underground is operating at maximum capacity, many bus service improvements have been undertaken, and central bus services are currently enjoying something of a resurgence. Although the rear-entrance double-deck AEC Routemaster is the archetypal London bus, they were withdrawn in the early 2000s owing to their age, their inability to comply with disability legislation or accept wheelchairs or pushchairs, and their requirement for a two-person crew. All other local bus services are now operated by modern low-floor buses, which may be single-deck or double-deck. Most buses operating in London have two sets of doors, and passengers board the bus using the front door and alight using the rear door, whilst some buses on less busy routes have only one door. Since 2006, all buses are low-floor and accessible, accepting passengers in wheelchairs and other mobility impaired passengers. Following the privatisation of London bus services in the 1990s, the operating contracts for local buses in London are subject to a system of competitive tender. A wide range of companies now operate bus routes across London. Many services have been contracted out to leading transport groups such as Abellio, Arriva, ComfortDelGro, Go-Ahead Group, RATP Group, Stagecoach and Transit Systems. Connex, FirstGroup, National Express and Transdev previously operated services in London. Limited stop / Express routes A small number of bus routes (namely routes 607, X26, X68 and X140) run a daily "limited-stop" service, travelling a long distance but serving few stops in its route. Other bus services Some local bus routes in the outer areas of London cross the London boundary. London Buses services that cross the boundary have standard red buses, and charge London fares for the whole length. Buses from outside London that cross into London are in their operators' own colour schemes, and may not accept the London fares even within the boundary. Privately run bus services may also be operated independently of the regulated London bus network, but still require a permit from TfL. This permit applies to any service which has a stop in London and another within 15 miles of Greater London, such as commuter coaches, school buses and supermarket shuttle buses. Night buses Night buses began running as early as 1913, and they form part of the London Buses network.