(no crew on transit units). Service may be on a fixed schedule or in response to a passenger-activated call button. Automated guideway transit includes personal rapid transit, group rapid transit, and people mover systems. Personal rapid transit (PRT), also called personal automated transport (PAT), is a public transportation concept that offers on-demand, non-stop transportation, using small, independent vehicles on a network of specially built guideways. People mover or automated people mover (APM) systems are fully automated, grade-separated mass transit systems which serve a relatively small area such as an airport, downtown district or theme park. The term "people mover" has become generic for the type of system, which may use technologies such as monorail, duorail, automated guideway transit or maglev. Monorail means a system of guided transit vehicles operating on or suspended from a single rail, beam, or tube. Usually they operate in trains. Monorails are distinguished from other types of elevated rail system by their use of only a single beam, and from light rail and tram systems by the fact they are always grade-separated from other vehicles and pedestrians. Suspension railway is a form of elevated monorail where the vehicle is suspended from a fixed track (as opposed to a cable used in aerial tramways), which is built above street level, over a river or canal, or an existing railway track. Service type Local service Means trains stop at every station on a route. For light rail vehicles operating on city streets as trams or streetcars, local service is analogous to local bus service, where stops are every block or two apart. Regional service Regional passenger trains may be classified as either slow or stopping trains, or else limited-stop or semi-fast or express trains, where not all stations and stops are served. For example, a pair of closely spaced trains may both stop at the most heavily used stations. For lesser-used stations, the first train stops at alternate stations, while the following train stops at the stations missed by the first train. Express service Means trains operate for long distances without stopping, skipping some stations between stops. This speeds up longer trips, especially in major urban areas. In major cities, express trains may have separate tracks for at least part of their routes. Passenger boarding Street-level boarding Used primarily by light rail and tram lines that stop on the street rather than at stations. No platforms are used,