The extreme cost of US projects is a mystery to me, here I hope BruceMcF could shed deeper insights. I can only speculate that the reasons are more extreme versions of what make some projects drawn-out and too expensive here:

• corrupt relationship between construction companies and city politicians
  
• local construction company protectionism (the local firm may not know efficient methods of construction)
  
• no experience with commanding big projects
  
• boosterism (too much care for flashy appearance, which may be expensive on itself and may require lots of legal work for permits or disputes)

It may be that wages are also a factor (European construction is full of low-wage immigrant workers, including illegal; though maybe that's less so in specialised rail construction). But what I noticed is that even the initial stages of major projects -- e.g. feasibility study, environmental impact and other preliminary studies, detailed planning -- are tendered for much higher sums in the US (up to an order of magnitude difference). I can't fully explain that even with all of the above listed reasons.

_{*Lunatic*, n.
One whose delusions are out of fashion.}