Well, marginal dollar on road yields greater than marginal dollar to economy. Current political structures in the US means that marginal dollar on passenger rail is absorbed by the agents in charge of ensuring the rail existence. i.e. the $500 million railway and the $1 billion version of (say) the Central Subway will both yield probably the same economic benefit. Rent-seekers know that they can extract a large amount of economic rent because of government propensity to spend and political reasons that require governments to chase sunk costs.

Because of the large amount of roadway in America, the number of parties you can call to get more road built is pretty big (as Thiel says, competition is the enemy of profit). And the primary place you put more road is out where there's nothing. Because of the existence of a large network of roads, you could build roads where there is nothing and it's easy to get to where there's something.

Personally far prefer public transit so I'm pretty interested in ideas that change the structure of the interactions to pursue efficient construction.