Give the Regional Transportation District credit for perseverance. The district insists on keeping the $4.7 billion FasTracks light-rail expansion on schedule, even though the surging costs of materials (for starters) could bust the project's budget.

Residents were promised a timely rollout when they approved the 0.4 percent sales tax hike to pay for FasTracks. It's what they deserve. Still, it's hard to see how RTD can stay on track without cutting even more corners.

That's why we're encouraged to see the district ask for a second opinion: Over the next few weeks it will bring in outside consultants with expertise in engineering and financing mass-transit projects. The goal is to seek ways to minimize building costs, bring more revenues to the system, or both.

The RTD board willing, private contractors will clearly play a larger role in the completion of FasTracks. And that's wise, because RTD has already had to pare back some of its plans.

The district has proposed more than $100 million in cost-cutting design changes for the West Corridor just to keep that line close to its $511.8 million budget cap. At some point, fewer amenities will make light rail less attractive to passengers, and increase headaches for motorists, too. That's a disservice to everyone.

Enter the private sector, which helped the T-REX road and rail project open ahead of schedule and on budget.

T-REX employed the method known as design-build. A single contractor handled engineering and building the project. Construction crews did not need to get engineers to approve a design change if an unforeseen setback occurred; the same company handled the entire operation, and could sign off on modifications more quickly.

RTD officials say they're open to a greater private sector role for FasTracks, possibly using the process known as design-build-operate-maintain.

Under that method, a single contractor or consortium would bid to not only take care of engineering and construction, as with T-REX, but also run and maintain the lines. The contractor would get its revenues mainly from transit taxes and fares.

When the contract ended, say in 15 or 20 years, RTD would get those segments of the rail system back.

The contractor would have to eat any cost overruns. RTD would retain oversight of the build-out and of fares once the lines started running. But the contractor and its investors would assume some of the risks.

In Europe and Asia, it's common for major public highways and transit systems to be leased or franchised to private companies using design- build-operate-maintain or similar arrangements. The idea is gaining currency stateside, too. The 20.6-mile Hudson-Bergen light-rail project in New Jersey used design-build-operate-maintain. Monorail systems in Las Vegas and Seattle have also employed this process.

If FasTracks cost projections really get out of hand, RTD might want to consider leasing existing light-rail lines. Indiana recently leased the state toll road that links Gary to Chicago for 75 years and nearly $4 billion to a consortium based in Spain and Australia; the same group has a 99-year, $1.8 billion lease for the Chicago Skyway, an elevated toll road on the city's south side.

There's no telling if any private operator would bid for, say, the Southwest light-rail line if it were available. But RTD should not exclude that option, so long as proper safeguards for taxpayers and transit riders remain in place - especially if the alternative is nickel-and-diming FasTracks.