Congratulations to Los Angeles on today’s opening of the Gold Line light rail extension, which runs from Union Station through several historically neglected suburbs to East Los Angeles, and will probably someday go further.

More precisely, congratulations to every Angeleno except Ari B.Bloomekatz of the Los Angeles Times, who wrote this:

Why is the Gold Line not a subway? From the beginning, residents and politicians on the Eastside pushed for the Gold Line extension to be built completely underground. In the end, transportation planners decided to make a roughly 1.7-mile portion of the Gold Line a subway — the part that runs underneath Boyle Heights. The majority of the route runs above ground.

Ah, those nasty cruel “transportation planners”! Sorry, but the answer to “why” is not “the planners decided …” unless your main goal as a journalist is to instill feelings of ignorant helplessness in your readers. Planners and political leaders made these decisions for a reason, and that reason is the real answer to the question. Mr. Bloomekatz missed an opportunity to educate his readers about the real choices that transit planning requires. We all need citizens to be smarter about those choices.

Yes, there was once a plan to extend the Red Line heavy-rail subway east from Union Station to cover a similar area, all underground. There is no way this would have cost less than $2 billion at today’s prices, probably much more. The Gold Line extension came in just under $0.9 billon, and that’s high largely because of the one underground section they still had to build, to get through a segment of Boyle Heights where there was no good surface option.

Los Angeles has the worst deficit in transit infrastructure of any city in North America. No other city on the continent grew so large without retaining and expanding its rapid transit. Today, most of the region’s leaders understand this was a mistake, and are trying to build rapid transit as fast as they can. But underground construction is massively more expensive, so if you insist on undergrounding everywhere, you’ll get a much smaller network.

When you’re trying to build as much of a network as you can, as fast as you can, there are just three technically compelling reasons to bear the huge cost of going underground:

to get past a specific surface obstacle that can’t be bridged over more cheaply, such as the Hollywood Hills, or San Francisco Bay, or more commonly a segment where there’s no credible surface alignment, such as through Boyle Heights on the Gold Line. (This is, admittedly, a grey area, as the credibility of a surface alignment often turns on the politics of how much you can impact current traffic and parking on the street.)

that can’t be bridged over more cheaply, such as the Hollywood Hills, or San Francisco Bay, or more commonly a segment where there’s no credible surface alignment, such as through Boyle Heights on the Gold Line. (This is, admittedly, a grey area, as the credibility of a surface alignment often turns on the politics of how much you can impact current traffic and parking on the street.) to get to a crucial station site — usually a connection point with other lines — where there’s no surface or elevated option.

— usually a connection point with other lines — where there’s no surface or elevated option. to serve a very dense corridor where highrise development is or will be the norm, and which can therefore generate the very high ridership to justify a subway, such as Wilshire Blvd. in Los Angeles, Second Avenue in New York. Even here, we’re talking about streets that are already so built out that elevated structure would be unacceptable. If you’re just talking about future highrise corridors, elevated may still be the answer, as it was in Vancouver.

(BART in San Francisco used to have a good reason: The original network offered to go underground for any city that wanted to pay the difference in cost itself. Only Berkeley accepted this offer, which is why BART is underground within its city limits. This elegant approach had the virtue of insisting on local funding to manage a local impact, where this impact was broadly viewed as acceptable by most of the other cities involved.)

Remember, too, that even if your current heavy rail subway is underground, it’s extensions don’t have to be. Most “subways” are heavy rail which take power from a third rail. Unlike light rail, these lines usually don’t intersect streets, but they can be elevated and often are, sometimes with not-bad visual impacts.

So congratulations to all but one of the people of Los Angeles. This is a good start for a long-neglected part of the city.

UPDATE: See follow-up post here.