Robust ridership projections that match the successful Hiawatha light-rail line, cohesive political support from five cities and a county, state and federal funding formula have made it more likely than ever that a southwest line will be built.

Key questions remain, however. Which route would best connect Minneapolis to Eden Prairie? And which proposed route would qualify for federal funding, which would pay half of the estimated $1 billion in construction costs.

The route through St. Louis Park, Hopkins and Minnetonka is generally set. And despite there being two possible routes in Eden Prairie, there is near unanimity that the line should connect the burgeoning business centers in the eastern part of that city, rather than use a less expensive, but less dense, residential section in the west.

The Minneapolis alignment decision is more problematic, and it has created a fundamental and healthy debate over the role of rail. One route, commonly called 3A, would run along the existing Kenilworth Trail on the west side of Cedar Lake and connect to the planned intermodal transit facility at the Twins new baseball stadium. The other option -- commonly called 3C -- would bring the train along the Midtown Greenway through Uptown and then head north, either via Nicollet, Blaisdell or 1st avenues. From there, a tunnel would be dug from the Greenway to Franklin Avenue in order to keep speeds competitive with route 3A. Once downtown, one 3C route would run down Nicollet Mall and meet the Hiawatha Line at 4th Street, but not directly connect to the intermodal station. An alternative would go along either 11th or 12th streets and connect at the planned transit hub.

While supporters of the C3 Uptown/Greenway route make a strong case that this portion of south Minneapolis should be included, the capital cost -- estimated at $1.5 billion to $1.8 billion -- would be significantly higher than the 3A Kenilworth route, which is estimated to cost $1.1 billion to $1.25 billion. Annual operating costs that would be $2 million to $6 million less than 3C also favor the 3A Kenilworth route.

Taking on those additional costs might make sense if ridership would be much higher on the 3C Greenway/Uptown route, but both 3A and 3C are projected to have 28,000-30,000 daily riders in 2030.

The ridership numbers are key because the cost-effectiveness index used to qualify for federal funding would be $28 to $31 for 3A Kenilworth, compared with $39 to $44 for the Nicollet version of 3C and $44 to $48 for the 11th Street version.

Paying more for the same ridership benefit would be a poor use of public money. And, crucially, only route 3A, using the Kenilworth Trail and connecting with the Hiawatha, Central Corridor and North Star lines at the ballpark, would likely qualify for federal funding, which is supposed to cover half the cost of the project. Without the federal money, southwest LRT would be literally and figuratively derailed.

Public officials should focus on the 3A route but also consider ways to serve transit demands in Uptown. One possibility is a Midtown Greenway trolley that would connect with the southwest and Hiawatha lines. A trolley line would be more compatible with bicycle and pedestrian use of the Greenway, and it could make more frequent stops than a light-rail line.

After years of study and debate, the 3A route has emerged as a clear and practical favorite. But as planning moves ahead, officials should keep in mind that creating an effective service alternative for Uptown would help accelerate economic development along the Greenway and Lake Street and more fully address the metro transit needs of this and future generations.