A shuttle bus service is a public transport bus service designed to quickly transport people between two points. Such a bus will 'shuttle' backwards and forwards between the two points, normally without any intermediate stops and with a high frequency of trips. Such services are often designed to serve specific customers, such as schools, hotels, attractions, park and ride routes or airport bus services. They may also be used to replace train service when rail lines are closed (for maintenance, or during emergencies). Unlike regular bus routes, shuttles are (usually, but not always) free of cost.