There's a lot to be said about light rail and streetcars; in this post I want to focus on how these two kinds of trolleys fit in with our goals (as articulated above), and particularly how they can get people out of their cars. If you're trying to decide which kind of train to push for, there are three major questions: (a) Can the train mix with cars and trucks? (b) Can the train mix with freight trains? (c) How long can the train be?
In contemporary usage, streetcars run for most of their length in mixed traffic. We don't have a good example here, but if you go to Girard Avenue in Philadelphia you can ride one. Light rail trains run for most of their length in their own right-of-way, but may cross many (or all) streets at grade. Good examples include the Hudson-Bergen Light Rail and Newark City Subway in New Jersey.
There are important differences between light rail, streetcars and other forms of rail. Elevated rail is raised above the street like the #7 train for most of its length, and subways go below the streets like the R train. Commuter rail can run elevated or in a cut like the LIRR Port Washington Line in Queens, but sometimes crosses streets and roads at grade like the LIRR Hempstead Branch in Garden City.
Commuter trains in the New York area typically share tracks with freight trains, which means that the Federal Railroad Administration requires them to be unnecessarily big and heavy. PATH trains run in subway tunnels in Manhattan, Jersey City and Hoboken, but they have a waiver from the FRA to run on shared track between Journal Square and Newark. Light rail and streetcar trains are typically not allowed on shared tracks, but the River Line between Trenton and Camden splits the time with the Chesapeake freight railroad: they run passenger service during days and evenings, and CSX runs freight overnight.
Because they mix with cars so much, streetcar trains are typically only one or two cars. Light rail trains sometimes have up to five or six cars. Subways and commuter trains can have ten or twelve cars.
Streetcars are often considered to be cheaper than light rail, and light rail to be cheaper than subways or commuter rail, but this is not magic. The cost comes primarily from grade separation and longer, heavier trains. This has important implications for transit, particularly in the New York area.
The first is that building the wrong system can limit your capacity. If you've got enough passengers to fill a four-car light rail train but you've only got a two-car streetcar, you're going to get crowding. Same if you've got enough passengers to fill a ten-car subway but they're trying to squeeze into a six-car light rail train.
The second is that building the wrong system can limit your speed. All other things being equal, a streetcar in mixed traffic is going to be slower than a light-rail or commuter train in a dedicated right-of-way with grade crossings, and a train with at-grade crossings is slower than one without them.
Both of these speed and capacity limitations have the effect of limiting how many passengers the line can carry, which can in turn limit revenue. They also decrease the value to passengers, which can drive them away. You may be able to pack those cars, but you'll have to keep the fare low to do it.
In a future post I'll discuss these factors in terms of specific projects here in the New York area.