Hard to say - I've seen a dozen different potential reasons. There are staffing agreements with labor unions that require 25 guys to operate a tunnel boring machine in NY that operates with 9 in Spain, and each sandhog costs 200k a year with pensions and healthcare contributions included. Buy America rules, obsolete and functionally useless safety rules, and tough environmental review standards drive up costs and keep us from buying rolling stock off the shelf. A lawsuit-happy legal system with strong protections for property rights leads to projects being over-engineered to avoid any impacts on the surrounding community that could encumber projects with legal challenges. A lack of in-house talent able to evaluate bids for projects and requiring everything to be outsourced, a lack of competitive bidding with frequent cost overruns, and duplicative layering of agencies at the local, regional, state, and federal level. Bundling tangentially related projects like pedestrian pathways, environmental restoration, and public art into transit projects (I'm not against these things but they should be funded and evaluated on their own merits by the competent authorities). Operating costs driven up by overstaffing - commuter trains that operate with just a driver in Europe have a driver and two conductors in NY, and labor is 2/3rds of the cost of operating a train. We also get less revenue hours out of each driver, even in busy systems like NYC. A political dichotomy where Republicans are generally anti-transit, rather than serving as an effective counterweight to the liberal impulse to push grand designs by pushing for efficiency like conservative parties do in other countries. It's a clusterfuck. I really like Alon Levy's blog for its extensive coverage of the cost issue in American transit - he looks at these issues from an international perspective that is missing from a lot of American transit advocates' writing.