Gov. Tim Pawlenty issued two stern messages last week to planners of the Central Corridor, the proposed light-rail line between downtown Minneapolis and downtown St. Paul. His directives: Spend no more than $840 million, and settle your differences on the design by Feb. 27.

The governor, a good student of group dynamics, knows that a firm deadline and bottom line help move committees toward decisions. His decree, bolstered by a threat to withdraw his support for the project, likely had that goal.

But the governor's directive was incomplete, perhaps because what he left unsaid goes without saying: The Central Corridor line is among the most significant pieces of public infrastructure that will be built in Minnesota in many decades. It ought to serve this region for the rest of this century -- without having to be ripped out and rebuilt because of design flaws evident from the start (witness Crosstown Hwy. 62.) It's crucial that the Central Corridor line be done right.

That imperative has to be balanced against a deadline and the bottom line, to arrive at the best decision for Minnesota. Flexibility on the part of all parties will be required. Fortunately, creative alternatives and cooperative attitudes have lately emerged that bode for a good design to be delivered on budget, and maybe even on time -- or at least, soon enough to meet the September deadline to apply for the Federal Transit Administration's final design process in 2009. We find these things encouraging:

• A tunnel through the east bank campus of the University of Minnesota, once considered a must by university officials, is now one of at least three proposals from that quarter. The cost of a tunnel spiked when the location of the new Gophers football stadium was factored into its design. A campus tunnel now carries a price tag in the $250 million range, and pushes the whole project as currently conceived past $1.1 billion. That number is too far from $840 million to fly.

The most intriguing of the new ideas involves taking most auto traffic off Washington Avenue through campus, and routing it instead along East River Road. That would create a transit-and-pedestrian mall, not unlike the Nicollet Mall, on a street that experiences Nicollet Mall-at-noon traffic levels before and after every class period. An engineering study, funded by the university, is exploring that notion's cost and feasibility. Its findings, due this spring, deserve serious consideration.

A strong case can be made against simply adding a train to Washington Avenue at grade. It would make an already congested corridor less navigable, and less safe. Another idea, running light rail through the heart of Dinkytown, would likely reduce ridership and make the train less serviceable for a major campus destination, the University of Minnesota Medical Center.

• At the other end of the line, in downtown St. Paul, rail planners still insist that the line must run all the way to Union Depot -- and they'd prefer to loop around to its south side, to facilitate connections with future inter-city and commuter rail lines.

Here, too, flexibility -- and patience -- are in order. The trains the planners want the Central Corridor to meet won't arrive for more than a decade. Extending the line to the depot's concourse level can wait until there's a train to meet there. If push comes to shove, the same can be said for taking the line all the way to the depot.

• To their credit, St. Paul officials have suggested engineering changes in how University Avenue is rebuilt along the line, shaving more than $20 million off initial cost estimates.

Community groups are right to urge that the new line aim to serve the seniors and low-income people who live along its path, not just riders who want a quick trip from one downtown to the other. But adding up to three more stops to the original plan for stops at Snelling, Lexington and Dale avenues seems excessive, at least in the project's initial phase. The strongest case is for adding a stop at Hamline Avenue, where a new SuperTarget store has added to a retail cluster that's a destination for both commuters and local residents.

Clearly, city, county and university officials are not refusing to compromise. But right now, they are missing crucial pieces of information -- most notably, the feasibility and cost of alternatives to a university-area tunnel. To do the job right, Central Corridor planners must gather and consider that information as soon as possible. Until then, flexibility will need to be shown on all sides, including Pawlenty's.