The persistent myth that it just costs more to build train lines in the U.S. than it does abroad is mostly bunk, a new analysis finds — but costs quickly balloon when we start building them underground, for reasons that researchers can’t yet fully explain.

In a preliminary analysis of more than 171 intracity light and heavy rail projects in America and abroad — one of the most comprehensive studies of transit construction costs to date — the Eno Center for Transportation found that the average U.S. rail project costs $107 million per kilometer of track, or about 22 percent less than comparable international projects. And contrary to popular belief, light rail that carries fewer passenger is actually not substantially cheaper to build than heavy rail that can move a lot of people and goods.