DENVER — One of the most ambitious one-time mass-transit projects in the nation's history, called FasTracks — $4.7 billion, 122 miles of passenger rail and a hectic construction schedule of only 12 years — was approved by voters in Denver in 2004, along with a regionwide sales tax to pay for it.

But these days, swagger has been replaced by supplication. Projected costs have ballooned to nearly $7 billion, and the system faces a budget gap of $2.2 billion, mainly because of a sharp drop in sales tax revenue.

Worse, most rail lines are still on the drawing board, and the 10,000 or so workers needed when construction reaches full swing beginning in 2011 have not yet been hired, muting the economic stimulus.

Voters could be presented with a proposed sales tax increase as early as next year to finish a system that remains mostly an idea.

But as Denver prepares to sell itself all over again to the public, money is only part of the new equation. Colorado ranks 10th in the nation in purchases of hybrid vehicles per thousand residents, according to R.L. Polk & Co., a research firm, Thus, transit could be a harder sell, some experts say, if people feel they are already going green by getting 50 to 100 miles on a gallon of gas.

James van Hemert, the executive director of the Rocky Mountain Land Use Institute, a research group at the University of Denver's law school, said that in persuading taxpayers to continue their support, FasTracks will need to talk about the high environmental cost of even the greenest cars, because those cars will still need expensive, energy-intensive highways to drive on.