Supporters of a regional mass transit plan that was unveiled Monday morning say they are confident a light-rail line could be operating in central Indiana within seven years, despite the steep cost to taxpayers.

Details of the 25-year plan, revealed at the Julia M. Carson Government Center, call for a light-rail line running from downtown Indianapolis north to Noblesville and south to Franklin as early as 2017.

The rail lines to Franklin and Noblesville would be built along existing track. If funding allows, another line could be added from downtown northwest to Zionsville.

But finding money for the $2.4 billion plan remains the biggest obstacle. At least half the cost would be covered by federal grants, with taxpayers in counties benefiting from the revamped system bearing at least some of the additional financial burden.

Indy Connect, a regional initiative to improve general transportation options, recommends the use of a referendum to ratify a local-option sales tax or income tax increase to support construction and operating costs. The study estimates residents would pay an average of $15 a month per household for 25 years to pay for the plan.

“That’s where the rubber hits the road,” said Lori Miser, executive director of the Indianapolis Metropolitan Planning Organization, which is part of Indy Connect.

Miser’s goal is to work with the Indiana Legislature this session to craft language for a referendum, which could be on county ballots as early as November 2011. The decision to place the referendum on the ballot would rest with the counties.

“I’m confident at least some of the counties in central Indiana will do that,” she said. “It just depends on who sees value for the money and how soon they want to see something happen.”

The plan revealed on Monday relies more on a regional bus system and on fewer light-rail lines than previously envisioned.

A light-rail line expected to run along Washington Street on the east and west sides of the city—perhaps extending to Indianapolis International Airport—is absent from the proposal and could be part of the next 25-year transportation plan. The line would not be operational until at least the late 2030s, Miser said.

Replacing the line is a cheaper alternative known as bus rapid transit, in which 56 miles of BRT service would be added to supplement the traditional bus system.

Lines would be added along Washington Street, 38th Street, Keystone Avenue, Meridian Street, College Avenue and Madison Avenue.

Also, according to the plan, a regional bus system would provide triple the number of services, including more routes and longer operating hours than IndyGo, with wait times of just 10 to 20 minutes instead of 30 to 60 minutes.

Miser lauded the plan as a good “starting point” despite the initial downsizing in rail service.

“We think BRT is a great lead-in to light rail,” she said. “[Central Indiana residents] definitely want transit, more service and more frequency. But it also needs to be more affordable.”

The push for rail transit comes amid growing highway congestion and concern about pollution.

A report released early this year from IndyConnect claims a light-rail system under the earlier plan could result in 4,500 new jobs and more than $27 billion in additional regional economic output. The report estimated a 4-percent increase in value of property near rail service in Marion County.

Planners will offer more information about the plan in public meetings scheduled through Nov. 18 at various locations.