A: Light rail doesn’t need as much separation as metrorail, and can fit in narrower spaces.

Question: Why do most cities that are building new rapid transit systems use light rail, instead of heavy metrorail?

Someone asked me that question over the weekend. Here is my answer:

It basically comes down to cost and flexibility. Light rail is cheaper and more flexible, so cities use it unless they absolutely need the higher capacity of metrorail.

Heavy metrorail is built to be 100% grade separated all the time. That means nothing ever crosses the tracks. If you want to cross the tracks (as a pedestrian or in a car), you need a bridge or tunnel on a different level so that you can do it while trains are using the tracks simultaneously. This makes metrorail very expensive, and limits where you can put rail lines. In very highly urbanized areas such as downtowns, your only options are basically in a subway or elevated above ground.

Light rail on the other hand works at-grade, which is to say you can allow streets, sidewalks, and pedestrians to cross the tracks. That makes it much cheaper, and much easier to locate/build. You can run light rail along any street you want, basically. This is especially beneficial in downtown areas, where instead of a massively expensive subway or el, you can simply run light rail on the surface. You might opt to give it a dedicated lane or you might opt to run it in a lane mixed with cars, but either way it’s easier and cheaper than a subway/el.

And of course, light rail can run fully grade-separated in a subway or an el if you want it to, it just isn’t required. So if you find that you need a metrorail-level of capacity for part of your line, but not for all of it, you can still use light rail. Metrorail doesn’t offer that kind of flexibility. It has to be grade-separated all the time.

So light rail is generally slower and has lower capacity, but is much easier to fit into cities and is much more affordable. So you would only go to the expense/trouble of building metrorail when you need that very highest level of capacity.

Think of metrorail as being like the transit equivalent of an interstate highway, where there are no traffic lights and there’s not supposed to be anything that impedes traffic. On the other hand, think of light rail as being more like the transit equivalent of a big arterial road, or like Fairfax County Parkway.