Zweisystem at Rail for the Valley has done the math to put together some really useful information. I’m just going to go ahead and rip off the relevant bits. Here’s a comparison of costs for various recent heavy rail projects with the average cost for three light rail projects.

The heavy rail:

Toulouse metro line B – CAD $161 million/km.

Turin metro extension – CAD $196 million/km.

Meteor Metro, Paris – CAD $290 million/km.

Singapore metro – CAD $420.7 million/km.

Jubilee Line extension, London – CAD $559 million/km.

The light rail:

The average cost for three newly built LRT lines in the UK: $16. 1 million/km.

It’s tough to know exactly what to make of these top-line comparisons, but obviously the take-home point is that light rail is an order of magnitude cheaper to build than heavy rail. That looks like a pretty good reason to go for light rail, no?

But I think we need to be careful not to conclude too much from this. While heavy rail has, uh, heavy financial costs that light rail doesn’t, light rail has other costs. In transit, as in life, you often get what you pay for.

Let me give just one important example: speed. Heavy rail systems are typically a lot faster than light rail systems. One obvious reason is that heavy rail systems are more often grade-separated. They’re underground, or they’re elevated, or if they’re on the surface they’re separated from traffic. And that last point a really important one. We can get bogged down in transit-nerd jargon and worry about exactly what does and doesn’t count as “grade separated”, but it’s easy to remember one important point: if you take a train out of traffic, it can go a lot faster. It’s not even an all-or-nothing thing. The more you can take the train out of traffic, the faster it’ll go!

Now, while it’s certainly possible to build light rail that’s completely separated from all traffic, cheap light rail systems typically run their trains through at least some traffic. Think of Toronto’s or Philadelphia’s trolleys. Or the cool-looking, sleek trolleys-of-the-future that run through Portland. These are trolleys that stop at red lights and get stuck behind cars turning. In other words, they don’t go much faster than the B-Lines here in the Lower Mainland. Which is to say, a little better than half the speed of the Skytrain.

To be sure, there are ways to build faster light rail. (They usually involve spending more money and disrupting a lot more traffic.) But grant for the sake of argument that one downside of light rail is that it’s slower than heavy rail alternatives. That means heavy rail is going to be the best option in some circumstances–even with its heavy price tag. Zweisystem’s thinking about the Valley, and I’m in no position to say what makes sense out there. But I know that Translink should be thinking hard about how to move as many people as possible–as fast as possible–from Commercial to the west side of the city, maybe all the way out to UBC. It’ll be worth it.