What is the difference between light rail, heavy rail, and commuter rail?
Definitions of the types of rail-passenger transportation services are complicated by the great variety of systems used throughout the world now and in the past. In the United States, the terms heavy rail and light rail are generally applied to two forms of urban rail systems that are not part of the national railroad network. Common characteristics of heavy-rail lines are exclusive rights-of-way, which may include subways, elevated structures, or ground-level sections; multiple-car trains propelled by electric motors drawing power from a third rail or overhead wires; and high-level platforms at all stations. In the MBTA system, the Red, Orange, and Blue lines are classified as heavy rail. Light-rail lines can operate on exclusive rights-of-way similar to those of heavy-rail lines, but often some segments of a route is on tracks located in city streets. They may have high-level or low-level station platforms, or a combination of the two. Most systems have electrically powered vehicles
