Costs are a major part of the problem. US rail construction costs are by far the most expensive in the world [1]. Other countries are able to build rail at costs in the $100-250 million per km range (even in dense cities). The East Side access project in NYC has costs around $4 billion per km. Los Angeles has much better costs in the $400-500 million per km range [2]. Its hard to imagine the US will be able to build much transit at those costs.

From 2012[3]: "When asked by transit blogger Benjamin Kabak about its high construction costs, Michael Horodniceanu, president of the New York City Metropolitan Transportation Authority’s capital construction division, gave a two-word answer: “work rules.” Citing the example of the city’s revered sandhogs, he said the MTA employs 25 for tunnel-boring machine work that Spain does with nine."

[1] https://pedestrianobservations.wordpress.com/2011/05/16/us-r... [2] https://pedestrianobservations.wordpress.com/category/transp... [3] http://www.bloombergview.com/articles/2012-08-27/labor-rules...