There is no standard definition, but in the U.S. (where the term was coined from the engineers term light rail in the 1970s), the term expressway operates mostly on dedicated rights-of-way, using individual streetcars or a combination of several units that forms one train, with lower capacity and slower speeds than passenger trains on longer heavy rail lines or subway systems. Generally, light rail is used to describe systems that are in some point on the spectrum between passenger trains, as far as capacity and speed. Light rail is a modernized version of streetcars or streetcars (American English) or trams (British English) in many localities, though the term is more consistently applied to contemporary or upgraded tram or trolley operations employing features most commonly associated with metro or underground operations, including exclusive rights-of-way, multi-unit train configurations, and signalling-driven operations.
According to this research paper, which documents the birth of light rail in America, the term light rail transit was introduced in 1972 to describe the new North American systems that were modeled on Germanys stadbahn system. The term is generally used to describe rail systems that have rapid transit-style features, usually using electric rail cars operating mostly on private rights-of-way, separated from other traffic, but occasionally mixed in with other traffic on urban streets when needed. There is a considerable overlap in the technologies, with many of the same vehicles being used for both, and it is usual to categorize streetcars/trams as a subtype of light rail, not a separate mode of transit.
The distinction is not always very helpful, with some light rail systems having capacities that are similar to larger heavy rail subways, and some so-called heavy rail systems having speeds comparable to those of streetcars. Ultra-light vehicles cannot, therefore, co-exist with heavy rail, or indeed with most light rail systems, because light-weight structures, compared with those of cars or buses, are insufficiently robust to withstand the impacts from conventional trains. In contrast, light rail vehicles can ride on trains carrying multiple cars carrying up to a theoretical 20,000 passengers an hour on far narrower rights-of-way, no wider than two-car lanes for a two-track system.
Because one light rail track can carry up to 20,000 passengers per hour, as opposed to 2,000-2,200 vehicles per hour for one highway lane, light rail can theoretically provide substantially greater potential for congestion reduction per dollar as an additional highway lane in congested urban areas. In terms of operating costs, each bus car requires a single driver, while a light rail train could carry three or four cars with far greater capacity on one train, controlled by one driver, or without drivers at all in fully automated systems, increasing labor costs for BRT systems as compared with LRT systems. Compared with single-occupancy cars, mass transit, particularly rail systems, is a far more efficient method for moving people throughout metropolitan areas.
Yet, since rail transit is the most effective public transit available, many metropolitan areas continue to build new rail systems or upgrade existing ones. Without being able to demonstrate a modern, effective rail system, cities frequently perceive themselves as falling behind in the development curve. The development, construction, and upgrade of public transportation systems–especially light rail–has become the primary focus for many cities, which view these systems as a panacea to these transportation-related problems (see glossary for definitions of light rail and other transportation terms).
In some cases, cities view rail transit system development as part of a larger development plan to attract industry. Light rail, or light rail transit (LRT), is a form of urban rail transportation which generally uses less large-scale equipment and infrastructure compared to bus rapid transit systems, and contemporary light rail vehicles generally travel on a rail line. Light rail vehicles (LRT) are distinguished from rapid rail transit (RRT) vehicles in their ability to operate through mixed-traffic, typically resulting in narrower car bodies and articulated joints in order to operate through the crowded street environment.
Docklands Light Rail in Calgary, Alberta uses a number of general-purpose LRT techniques in an effort to reduce costs, including minimising both below-ground and above-ground tracks, sharing transit malls with buses, leasing rights-of-way from freight railways, and combining the building of an LRT with freeway widening. The Docklands Light Rail uses a reversed third rail for electrical power, allowing the electrified tracks to be covered with the electrical energy drawn from underneath.
The development of technologies for catenary-free, low-floor streetcars has made it possible to build such hybrid systems with only short, shallow sections of the sub-surface beneath key junctions, because the required height of the crossings can be reduced considerably in comparison with normal light rail vehicles. All new light rail vehicles, except grade-access vehicles, covered by this subsection must provide an attitude-change mechanism or access equipment (e.g., elevators, ramps, or deck plates) that meets paragraphs (b) or (c) of this subsection, and clearances sufficient for at least two users in wheelchairs or mobility assistance vehicles to access areas, each having minimum clear floor area of 48 inches by 30 inches, that does not unduly limit the ridership flow. All new light rail vehicles, other than level entry vehicles, covered by this subpart shall provide a level-change mechanism or boarding device (e.g., lift, ramp or bridge plate) complying with either paragraph (b) or (c) ; of this section and sufficient clearances to permit at least two wheelchair or mobility aid users to reach areas, each with a minimum clear floor space of 48 inches by 30 inches, which do not unduly restrict passenger flow. If a wheelchair or mobility device-using passenger cannot fit in the same vehicle.
Standing passengers in an SLRV can almost double the carrying capacity. The SLRV — designed in collaboration with Osaka, Japan-based train vehicle maker Kinkisharyo — features a standing-level entry, allowing passengers with disabilities — as well as those using strollers, bikes, and similar devices — to walk or roll right into a train, rather than using mechanical lifts. The new modified vehicles began service June 23, 2008, with Car No.151. Dubbed the Super Light Rail Vehicles (SLRVs) due to its greater length and increased passenger capacity, SLRVs will carry about 100 passengers, up from the 75 passengers of the existing vehicles. Special Use Platforms, orHigh Blocks, that had served people with disabilities and mobility issues since the Rail System opened in 1996, also will be removed from service, as those passengers begin to utilize lower-floor Center Doors on every trains mid-section. At the lower end of the spectrum for passenger rail are streetcars and trams, while at the higher end are high-capacity rail systems such as New Yorks subways or DCs subway.