The elected board of the Regional Transportation District will face some difficult choices over the next couple of years as it tries to keep the FasTracks rail expansion, as RTD said in May 2007, "within the general scope" of the project promised taxpayers in 2004.

As construction costs have surged - the state's Construction Cost Index soared by 52 percent in 2005 alone - RTD has looked at a series of cost-cutting moves to keep the buildout on schedule for completion in 2018 without having to ask voters for more money or shorten one or more rail lines.

The board remains committed to full buildout. Even so, fiscal realities appear poised to force it to consider unpleasant options. Such as: Postpone the completion of FasTracks; shorten low-ridership corridors; abandon one or more of the corridors; or seek a tax hike.

The fourth alternative should be off the table entirely. Metro area voters have already approved higher sales taxes for FasTracks - to the point that mass transit consumes a heavily disproportionate share of total transportation funding in the metro area. This week, however, the mayors of Arvada, Aurora, Denver, Lakewood and Longmont stated they would oppose the first three options. (They said they weren't crazy about a tax increase, either, although they didn't rule it out.)

So what to do? Maybe there's a fifth option. If the board wants to keep faith with the public, it could more aggressively pursue shifting the design and construction of the transit expansion to private contractors - a process RTD and the Colorado Department of Transportation used with great success in the T-REX project. Studies suggest successful deployment of these public-private partnerships could cut construction costs by as much as a third in the corridors they're used.

Such public-private partnerships are already under discussion for two of FasTracks' four commuter-rail lines. They should also be considered for the two other commuter lines, as well as the Interstate 225 light rail and the U.S. 36 Bus Rapid Transit corridors.

The board should also investigate the potential savings of fully leasing corridors to private operators, even if that means renegotiating contracts with union light-rail and bus drivers.

Postponing the full buildout of the corridors that are projected to have lower ridership, or that may duplicate other transit services, should be on the table as well. Portions of FasTracks that will get the heaviest use and relieve commuter congestion should be the priority.

We see several lines that could be delayed so the core of the FasTracks network isn't compromised:

* The Northwest Corridor basically parallels the U.S. 36 Bus Rapid Transit Corridor from Union Station to Boulder. And the most heavily subsidized segment of FasTracks by far will be the final stretch of the northwest line, from Boulder to Longmont.

* The portion of the East Corridor from I-225 to Denver International Airport will largely handle airport traffic. Connecting the east line with the I-225 Corridor should be the priority.

* The Gold Line to Ward Road in Arvada.

As costs escalate, it will become increasingly unrealistic for RTD to pretend it can be all things to all people - unless of course it has a couple of miracles up its sleeve. Otherwise, it's time for the RTD board to face the hard choices - and then to make some.