One of my rules of thumb whenever I see something sort of odd that has no obvious explanation is: ADA. That is, it might very well be the result of someone trying to comply with the Americans With Disabilities Act. To see what I mean, Matt Yglesias has a fine little story here about how ADA compliance morphed into a $7 billion track renovation project in Washington DC.
In fairness, ADA is only a part of the story. Still, it's a part. And it's surprising how often it seems to play a role in otherwise inexplicable projects and rules. What's more, even if ADA isn't truly the taproot of all this proposed new construction at Union Station, it's still a pretty good story about how a relatively small improvement in an already densely-built urban area leads by inevitable concatenation into a fantastically complicated and expensive project. For want of a nail etc.