Heavy rail (may be called "rapid transit", or simply "metro") is a form of rail transit generally found in the largest cities in the United States. "Heavy" rail implies the system's ability to handle large loads of riders, not necessarily the weight of the trains. Due to high costs of construction for the entire system, many cities chose light rail technology that give them more alignment options (including subways commonly associated with heavy rail).
The size and scope of the system along with station spacing determine whether such system is local transit or regional transit. Large regional systems may have closely spaced stops in the urban core.
- Trains operate in dedicated environments (subways, elevated structure, or closed surface tracks) generally without grade crossings. This environment ensures higher speed and high frequency service free of outside interference.
- To handle large loads rapidly, heavy rail enforce and process fares at stations with faregates and feature level boarding.
- Trains are electric powered for fast performance and ability to run in subways.
- Trains tend to offer fewer seats and large standing room onboard to handle regular crush load.
While level boarding is common on heavy rail, not all heavy rail systems are fully accessible due to the lack of accessible route between the street and platform at some stations. These stations (built before 1970s) may have stairs and escalators but no elevators.