The city's release last week of the first set of preliminary numbers (cost and ridership) for a proposed rail system for Honolulu begins to put a face on what has been thus far a fairly vague concept.

But the numbers are only preliminary and almost surely do not represent the final accounting on this project. It is certain that the final tab will be higher for a rail system (if that is chosen) than the $3 billion announced last week.

For that $3 billion-plus, planners say, Honolulu would get a system that is expected to "board" between 120,000 and 150,000 people a day by the year 2030.

(By comparison, the existing bus system boards about 250,000 a day today).

There are a number of reasons the $3 billion and 120,000-150,000 ridership numbers represent only a first tentative look at the project.

For starters, no specific route has been selected, nor has a technology been identified. It could be light rail, modified heavy rail, a monorail, magnetic levitation or even a technology not thus far identified.

Then, too, the cost estimate only includes construction costs for the rail line and stations. It does not include the rolling stock, land acquisition costs and other factors.

And these numbers, as preliminary as they are, have not been stacked up side-by-side with cost and ridership projections for other alternatives, including expanding the existing bus system, creating a "managed lane" or "HOT" lane alternative or simply doing nothing at all.

That last alternative surely must be taken off the table.

While an alternative transportation system will not "cure" our gridlock problems, it will undoubtedly ease congestion. And as the city continues to grow and expand, the need for a good alternative for the car-and-highway system we use now will become increasingly critical.

Then, too, the costs — while appearing astronomical — should be offset by a number of factors. Making it easier for folks to get to and from work and school will increase productivity, which should be part of the final calculation. So, too, the cost savings of avoiding individual automobile trips.

And some of the construction costs might be offset by public-private partnerships (a thought offered by Mayor Mufi Hannemann) in which developers would construct stations in exchange for development rights around the stop.

Other jurisdictions have used this approach successfully.

It will take a great deal more work before the true costs and opportunities of various options are known and understood. But those true costs must be identified as clearly and honestly as possible before the public and the City Council are asked to sign off on this ambitious project.