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 if the line is closed due to rail maintenance, a breakdown of a train, a rail accident, strike action, or if the rail service is not economically viable.
Terms for a rail replacement bus service include bustitution (a portmanteau of the words "bus" and "substitution", may also be bustitute) and bus bridge. Substitution of rail services by buses can be unpopular and subject to criticism, so the term bustitution is often used pejoratively.
In Australia, a permanent or temporary rail-replacement service change is often referred to as bustitution.
In November 1941, the Western Australian Government Railways introduced its first rail replacement service, operating a service from Perth to Kojonup via Boddington. By 1949, there were 28 buses, and by 1959, more than fifty.
On the Queensland Rail network, to relieve congestion on the single track Sunshine Coast line, the rail service is supplemented by a bus service operated by Kangaroo Bus Lines on weekdays between Caboolture and Nambour as route 649. NSW TrainLink, Transwa and V/Line all introduced extensive networks in New South Wales, Western Australia and Victoria in the 1970s and 1980s that replaced regional trains.
As in the United Kingdom buses replaced rail services on closed lines. The most recent example can be found in County Wexford whereupon the suspension of rail services between Rosslare Europort and Waterford in 2010 Bus Éireann route 370 was introduced. However the bus takes considerably longer than the train journey and fails to serve Waterford railway station.
Bus have been used to replace rail in Japan when rail service have to be suspended due to disaster, accident, economics, or engineering works. Notably, in some cases where those rail lines are closed permanently, some of the former rail right-of-way are converted into bus right-of-way to provide grade-separated Bus Rapid Transit service.
When train services operated by Transdev in Auckland train services are sometimes replaced by a bus, the resulting service is called Rail Bus. Historically, New Zealand Railways Road Services replaced many train routes with buses.
During the British Railways Board's railway rationalisation in the 1960s, known as the Beeching cuts, 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.
Rail replacement bus services have been used to operate Parliamentary train services.
Rail-replacement bus services are common among urban rail transit systems, mainly due to unexpected service disruptions. For example, one of the effects of Hurricane Sandy in New York was that the New York City Subway required replacement bus service for several subway lines. As the subway runs 24/7/365, replacement bus service is also provided when subway lines were closed for regularly scheduled maintenance, so interruptions in subway service require replacement bus service, even during off-peak hours.
Planning rail-replacement services in a high-patronage environment, such as a high-capacity rapid transit network, requires efficient use and management of time and resources in order to prevent major travel disruptions. This was exemplified by a July 2015 shutdown on the Toronto subway during rush hour caused by a communication system breakdown, in which the local transit operator opted not to use replacement buses as "it wasn't possible to replace the entire subway's capacity with buses".  A similar incident as Toronto happened in Singapore on 7 July 2015 after a mass shutdown on the North South East West Lines after a power system failure. Operator SMRT and rival SBS Transit did not activate bus bridging but made all buses free islandwide due to the sheer scale of the disruptions. The Land Transport Authority made travel free available for any bus services passing MRT stations affected during any train disruptions and in the event a massive disruption affecting at least 2 lines, bus travel islandwide would be free..