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Friday, February 27, 2015

What is Light Rail Transit?




Almost all transit built in the United States since 1980 has been ‘light rail transit’ (LRT)--but ‘light rail’ is a regulatory classification for a vehicle, not a type of transit. The term ‘light rail transit’ covers a wide range of types of propulsion, guideway, and operating characteristics. A clearer articulation of the characteristics of different light rail systems would help to better determine the relationship between transit investment and associated outcomes.

The US is slowly emerging from a transit Dark Age. The combination of subsidized competition and public dis-investment effectively expurgated most urban passenger rail systems from existence. It is necessary to re-invent, or rediscover, the constitutive elements of effect transit systems. Historically, there have been many different types of rail transit systems, but they fall into a small number of distinct types, which might be called suburban railways, street railways, rapid transit systems, and tram-trains.

Suburban railways link major cities to suburban locations distant from the central city. They linked independent towns to a nearby major city. Suburban railways are characterized by high speeds and a very limited number of stops, and characteristically bypassed the outer edges of an urbanized area to reach a central location. The require exclusive guide-way, but their relatively infrequent operation (hourly) means that this requirement can be met through temporary physical separation (crossing gates) rather than the full cost of grade separation.

Street railways develop from the horse-cart service. Initially steam powered, the nuisance presented by the smoke and cinders meant that almost all systems were eventually electrified. Acting more as labor-saving devices, they acted as a ‘pedestrian extender’, rarely traveling much faster than a running man, and stopping frequently. In Europe, the modern implementation is known as a ‘Tram’; in America, as a ‘Streetcar’.

Rapid Transit systems represent yielding to necessity. In locations where cost or difficulty of obtaining sufficient surface right of way (viz: London, NYC, Chicago) underground or elevated construction was necessary. Full grade eliminates the need for sudden stops, making much higher speeds possible. The lack of cross-traffic also makes much higher headways possible, so that it is possible to run a very large number of trains on the same section of track. A bare handful of metropolitan areas have been able to fund the construction of new rapid transit corridors.

Tram-trains operate as railways outside of cities, but as street railways within them. Adapted from the ‘City-Rail’ system in Karlsruhe, Germany, it represents the revival of a historical form of American transit, the ‘Inter-urban’. Inter-urban’s were passenger rail lines running along freight railway right of way between cities, but on city streets within the city. They represent an effort to reduce the friction of transportation, by obviating the need for a mode-switch from train to tram at the edge of the dense, pedestrian oriented central core.


Every implementation of mass transit system represents a trade-off between cost, speed, and access. Evidence suggests that the trade-offs are not entirely granular, but rather represent a series of ‘market niches’ or functional classes of transit. The same classes of transit emerge independently, repeatedly, in different locations, at different times. Analyzing the characteristics of light rail determines that there are ‘functional classes’ of LRT, with different service characteristics, which resemble the service characteristics of historic transit modes.