In North America, the word downtown invites us to imagine the densest and most walkable part of any city, the place where transit and other non-car modes naturally thrive more than anywhere else. And where this is actually true, it's logical for all kinds of intercity and local transit services to focus there.

But when we project this model of downtown onto every city, we encounter fatal confusions. Downtown implies a single place; there's just one per city or metro area. But some cities aren't like that. Los Angeles and Houston, two take two famous examples, have a place called downtown, but it's really just a slightly larger cluster of towers among many clusters of towers dotted across the region. Downtown in this model is not like a center of energy around which the whole city revolves. It's like the brightest of a bunch of stars in a constellation, and not even the brightest by much.

So if you cling to the notion that downtown means "focal point of travel demand and especially transit", then you have to embrace the concept that an urban area may be a constellation of many downtowns. This is not just an American sunbelt idea. Most of the population of the Netherlands is in a single metro area called the Randstad; it's made of many small cities each with its own downtown, with a shared airport near its centroid. Many Asian cities are so uniformly and extremely dense that almost any point in the city could be called downtown, so long as there is some mixture of uses. All of Paris is about equally dense, with government, business, and retail districts all over the city and the tallest towers around the edge of it, so good luck finding "downtown" there.

In Citylab, Eric Jaffe gives us the supposedly bad news that the proposed Dallas-Houston High Speed Rail (HSR) line won't go to "downtown" Houston. Instead it will end at Northwest Mall, just outside the I-610 loop in the northwest of the city.

But most of the Houston transit-advocates I've talked with aren't sounding nearly as upset. That's because:

the proposed terminal is close to the centroid of Houston as a whole. It's also very close to Uptown-Galleria, the region's second downtown, and to Northwest Transit Center, the busiest transit hub in the western 2/3 of the city.

the terminal station area is massively redevelopable. You could easily build yet another downtown there, and if HSR is built, they probably will.

the project will provide great impetus for light rail or Bus Rapid Transit linking the station to the original downtown. These projects have been sketched many times and could include either I-10 nonstop links or a refurbishment of Washington Street, a promising old streetcar street linking the two nodes.

in high speed rail, the cost of the last miles into an historic downtown can be a huge part of the cost and grief of the whole project. So if you want high-speed rail to happen at all, provoking this battle is not always a sensible part of Phase 1.

The bigger challenge, for folks from strongly single-centered cities, is to notice the limits of the term downtown. As cities grow, there is no correlation between the sustainability of a city and its single-centeredness. On the contrary, single-centered cities present huge problems for transportation, because they use capacity so inefficiently. New York, for example, is spending over $10 billion on a project to fit more Long Island commuter trains into Manhattan, and to put them closer to jobs there. The demand is mostly one-way, so this requires either storing trains all day on expensive Manhattan real estate, or running them all empty in the reverse-peak direction. It's very inefficient compared to the transit problem in a multi-centered place like Paris or Los Angeles, where demand is flowing two-way most of the time.

So growing a single downtown isn't the key to becoming a great transit city. Quite the opposite, it's best to have a pattern of many centers, all generating high demand, and supporting balanced two-way flows between them that let us move more people on less infrastructure. This is the great advantage of Paris or Los Angeles or the Dutch Randstad over Chicago or Manhattan.

So remember: when it comes to the efficiency and abundance of transit — or roads for that matter — "downtown" isn't all it's cracked up to be. For transit, big clumps of density and walkability are great, but several are better than one.