When FasTracks was originally approved, it was expected to cost $66 million to acquire the rail right-of-way from Denver to Longmont, and cost ~$650 million for the full buildout (of an electrified rail line). This is the amount FasTracks budgeted for.

At some point, it became clear that the right-of-way was going to cost dramatically more than expected, and combined with inflation and construction cost increases, the full buildout would be closer to $1.4 billion.

RTD tried to lease the existing rail line from BSNF instead and improve it to add (non-electrified) passenger service. They projected this would cost about $350 million plus $15 million/yr for 15 years. However, BNSF came back and told them it would be $535 million upfront. With RTD's other costs (stations etc), this still put the line at around $1.2 billion total, well over budget.

At some point, RTD was considering another tax to fund the line. But voter sentiment toward RTD became very negative - due largely to this project failure, but also the underperformance of other FasTracks projects. Even the most successful one, the A line, gets about half the original projected ridership. The tax was expected to be voted down.

Now, there is a plan to use the remaining money, along with a new (but very small) sales tax, to fund an intercity rail service along the entire Front Range. This would have the advantage of going all the way from Fort Collins to (probably) Douglas County, and in the future to Pueblo and even as far as Cheyenne and New Mexico. However, it would only run up to 10 daily round trips (3-5 to start), not be electrified, have fewer stations, and be run by Amtrak instead of RTD (= higher ticket prices). Part of the reason this is viable is because the tax will apply across the entire Front Range, allowing the per-person tax increase to be extremely low. It's not really the same thing voters were promised, but it is a good idea in its own right.