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.
Planning rail replacement services in a high patronage environment, such as the high-capacity transit network, requires efficient use and management of time and resources in order to prevent major travel disruptions. On 7 July 2015, there was a mass MRT shutdown on North South and East West lines after power failure. Operator SMRT and rival SBS Transit did not activate rail replacements but made all buses free islandwide due to the scale of major disruptions. The Land Transport Authority made the transport free for any bus services passing by MRT stations during any train disruptions and in the event of massive disruption affecting more than 2 lines, bus travel islandwide would be free.
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