When the right solution to a problem is obvious but out of reach, it's hard to settle for second best. That's the predicament the Metropolitan Council finds itself in today -- its self-imposed deadline for a vote on the Central Corridor light-rail line's route between downtown Minneapolis and downtown St. Paul.

The right thing to do is put the train in a tunnel through the heart of the University of Minnesota campus. Washington Avenue is too congested already -- especially between class periods -- to accommodate rail at grade. Yet maddeningly inflexibile federal rules apparently deem a tunnel too expensive to win the $450 million in U.S. government financing that's essential to the project.

Met Council chair Peter Bell says a search for wiggle room in the federal rules came up empty. That means the council has no choice but to act on the only affordable option identified to date: converting Washington Avenue into an auto-free, transit-and-pedestrian mall.

Making the most of that option, and fairly fixing responsibility for its costs, must dominate the next six months for the council, its engineering staff and university infrastructure stewards. It's up to them to find a design that avoids sending thousands of additional vehicles through residential neighborhoods adjacent to the campus.

But to the extent possible, the Met Council should keep a door open for an 11th-hour shift to a third route option, via Dinkytown. When a tunnel was ruled out, rail through the Dinkytown trench alongside the existing rail lines and around the new stadium became university officials' preferred choice. A university-financed analysis of whether that option would meet federal requirements is expected to be completed in a few months. It's worth the wait.

Traffic-sensitive neighborhood and business groups near the campus are rooting for a Dinkytown route. Those for whom the new rail line will be a near neighbor for decades to come deserve a full exploration of the Dinkytown option before the formal application for federal money is made in September.

Keeping that option alive while pressing ahead with the design of a Washington Avenue transit mall prolongs uncertainty for Central Corridor planners, at a time when they are understandably eager for firm decisions. But some duplication of effort in coming months is a small price to pay if it leads to a design that works for the heaviest transit user in the state, the University of Minnesota.