A mode of transportation is a way of moving people or goods from one place to another. The broadest of these are land, water and air, each of which can be subdivided into more specific modes. Each mode generally operates in a different environment and uses a different technology.
Land transportation, by far the most common of the three basic modes, consists mainly of road transport and rail transport, although there are additional categories such as pipelines and cableways.
Urban passenger transportation can be broadly divided into the modes automobile, transit, walking, and personal two-wheel vehicles such as bicycles, scooters and motorcycles. Transit, in turn, can be subdivided mainly into bus and rail.
Urban passenger rail transport, in turn, is commonly subdivided into tram or streetcar, light rail, rapid transit, commuter rail, funiculars, monorails and cableways. The term heavy rail is also sometimes used to refer to either commuter rail or rapid transit.
The distinctions between some of these modes are not always clear. For example, the main differences between streetcar systems and light rail systems is that the former usually operate mostly in streets mixed with other traffic, whereas the latter are designed to have minimal operation in streets and fewer stops than streetcar systems in order to allow faster speeds. However, many streetcar systems have sections with some light rail characteristics, and in some cases light rail vehicles travel on streetcar system tracks along with streetcars for portions of their journey.
Another example of the often indistinct boundaries between modes is the increasingly popular tram-train concept. In this system, light rail vehicles operate over existing streetcar and/or light rail tracks in central urban areas but then switch to running on conventional heavy rail railroad tracks in outer areas.