Seattle's modest light rail system is going to end up costing $50 billion. Ultimately Americans are just really bad at infrastructure and if you're not willing to spend 10x what other developed countries are you're not getting it.
My understanding is that the original idea was to do a partnership / use BNSF rails (for some reasonably "small" fee [I think I heard $30-40 mil]) which were preexisting but then BNSF decided that the rail could only be used during certain time windows when it wouldn't effect their industrial traffic; OR RTD could pay something like $550 million. Not sure if the 23 years is to collect the tax for that cost or what.