A rail replacement bus service uses buses to replace a passenger train service either on a temporary or permanent basis. The train service that is replaced may be of any type such as light rail, tram, streetcar, commuter rail, regional rail or heavy rail intercity passenger service. The rail service may be replaced due to a breakdown of a train, a rail accident or closure for rail maintenance, or because the rail service is not economically viable. Terms for a rail replacement bus service include bustitution (a portmanteau of the words "bus" and "substitution") and bus bridge.
During British Railways Board railway rationalisation in the 1960s known as the Beeching Axe, bus substitution was an official policy for replacing train services on closed lines. This policy was largely unsuccessful, however, as the bus services were usually far slower than the train services they replaced, causing many passengers to give up on public transport altogether.
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