[mbcrower]

Highways are only useful because there are so many of them that connect just about every community in the contiguous US - what economists call a "network effect". Therefore adding a new highway tends to make the other highways and roads it's connected to more useful. Likewise, public transit systems like the NYC subway are useful because there are so many of them.

When the first train is built in a metro area, it's not going to be useful to very many people. As more lines are built, however, that first train line becomes useful to more people because they can transfer to another line, or a bus line. As the third and fourth lines are built, the marginal utility of that each new mile of tracks goes up as the train becomes feasible for more people.

Of course there are examples of both highways and trains that probably shouldn't have been built where they were built (http://streets.mn/2014/07/07/strangulation-on-the-green-line...). My point is that cost comparisons of building a new highway versus building a new train are skewed by the fact that most metro areas have a mature (even crumbling...but that's another point) highway system, but few is the US have a large network of public transit yet, other than buses that are often slow, late and dirty.