New York City is currently in the process of constructing a $4.5 billion Second Avenue subway line. The project is important: it will deliver much-needed relief to the overcrowded Lexington Avenue subway line in a city that is utterly dependent on mass transit to move people around.

But is the multibillion-dollar subway line actually necessary? There's a strong case for adding a second subway line to the Upper East Side, but Yonah Freemark points out that in terms of pure congestion relief one could accomplish almost as much by adopting a different form of international best practice — open gangway railcars, a technology that's been standard on foreign systems for over a decade yet is virtually unknown in the United States.

The benefits of open gangways

American metro trains are composed of separate cars that don't directly connect to one another. Consequently, passengers cannot safely move from one car to another without first exiting at a station. By contrast, in an open gangway system the cars are linked in a way that allows riders to safely step from one car to the next.

This has two main benefits from the standpoint of system capacity:

Riders can redistribute themselves from crowded cars to uncrowded ones, ensuring that all the available space is used.

A far greater share of the train's total length is available for passenger use.

In New York right now, about 9 percent of the length of Lexington Avenue trains is the space between the cars where nobody can ride. The increase in capacity due to improving the rolling stock is almost equal to the 14 percent reduction in crowding promised by the Second Avenue subway. Obviously, building an additional subway line has some advantages beyond congestion — it'll be much more convenient for people who live on Second Avenue or points east — but elsewhere around the country there are many lines that could use more capacity but don't have the population density to support an entirely new parallel tunnel.

Open gangways are now standard internationally

Of course, simply replacing all the cars in a major metro system would itself be very expensive. Indeed, it would be downright wasteful. But every so often, every city needs to order some new cars. This leads to a slow but steady replacement of older equipment with newer vehicles. And in 75 percent of non-US systems, there are now at least some open gangway trains plying the rails.

They are in brand new systems in China and very old systems in Germany. They are everywhere. Because the overall replacement process is slow, it's easy to miss how widespread open gangways have become as a procurement priority. But it's fair to say that they have become the standard in new mass transit railcars.

Meanwhile, in the United States, they are used in Honolulu and ... nowhere else. And no cities have plans to introduce them.

America's costly transit isolationism

Freemark runs through various technical objections to the use of open gangway cars in the United States and finds them wanting. If this were a single example, it would be tempting to give American transit operators the benefit of the doubt and assume they must have some good reason for rejecting this technology. But the truth is that it's just a small part of a wider and troubling trend of mass transit isolationism.

Everyone knows European cities have, in general, superior mass transit to American ones.

But American agencies are remarkably resistant to the idea that they should try to adopt some best operational practices from abroad. Amtrak has passengers board trains in a bizarre manner, America builds brand new mixed-traffic streetcars that are universally rejected abroad, and America rarely uses proof-of-payment fare-collection systems.

It's understandable that the United States, which is very large, tends to be somewhat insular across a wide range of topics. But when it comes to mass transit when we are so clearly not the world leader in usage or performance, cutting ourselves off from the rest of the world is a costly error.