Capital Metro envisions a multibillion-dollar light rail system for the future of transit in Austin.

Cap Metro staff are calling for creating a light rail transit system that includes a downtown subway system, underground city tunnels, at least one new bridge across Lady Bird Lake and a rail line to Austin-Bergstrom International Airport. Such projects are expected to trigger a record-shattering tax or bond election.

The exact cost, however, is unclear. Cap Metro's Project Connect program manager David Couch would not provide a precise price range. That likely will be revealed at Austin Central Library on Monday, when transit authority staffers make a recommendation at a joint meeting of the Austin City Council and Cap Metro’s board.

But with the commitment to light rail, expect the total cost to be closer to the $9.8 billion upper end of an earlier estimated cost range rather an estimate nearer to the $4.7 billion lower end, Couch said.

The most eye-popping feature of Cap Metro’s recommendation is the downtown subway system, which proposes tunnels under Guadalupe Street from Cesar Chavez Street to at least 14th Street; under Fourth Street from Republic Square Park to Trinity Street; and under Trinity Street from Fourth Street to Lady Bird Lake.

It also could include a tunnel under Lady Bird Lake parallel to the First Street Bridge and underground train stations at multiple locations, including Republic Square Park and Auditorium Shores.

Cap Metro will recommend creating the Orange Line light rail line, which initially will run along Lamar Boulevard near U.S. 183, down Guadalupe Street adjacent to the University of Texas and on Congress Avenue in South Austin to Stassney Lane. Cap Metro eventually would extend the route to north to Tech Ridge and south to Slaughter Lane in the 2040s, Couch said.

The transit authority also recommended an altered Blue Line light rail transit route running from Austin-Bergstrom International Airport to Riverside Drive, across a new rail bridge over Lady Bird Lake from the American-Statesman’s property near Congress Avenue and Riverside to Trinity Street.

But the Blue Line would not extend to Austin Community College’s Highland campus, as first proposed. Cap Metro suggested replacing that portion with dedicated bus lanes they are calling the Gold Line.

Instead, the Blue Line will connect with the Orange Line’s route, doubling the frequency of trains along the route in the northern half of the city’s core.

Financing such a system is the biggest question mark. The City Council ultimately will decide how taxpayer money would pay for the system.

The council likely will put the question to voters in November as either a bond election, a tax rate election or a combination of both.

Bond elections traditionally are used to finance substantial local infrastructure projects. However, the city is flirting with holding a tax rate election — a requirement under state law if the city wishes to increase property tax revenue by more than 3.5% — to pay for the system’s maintenance and operations.

The Federal Transit Administration requires a steady local funding source for the maintenance of tracks for access to federal money. Cap Metro estimates the federal agency could pay for 40% of the system’s construction.

The plan also includes creating new MetroRapid bus routes, converting its entire fleet to zero emission electric buses and building 14 park-and-ride lots throughout the city.

"These recommendations are a bold vision for moving people today and planning for the future of our growing region," Cap Metro President Randy Clarke said in a statement.

"Our community made it clear that they are ready for action," Clarke continued, "and this plan is the beginning of a mobility transformation that will improve the quality of life for everyone."

Cap Metro will take public input on its recommendation during the next two months. In May, the transit authority’s board and the City Council are set to finalize plans on financing the system.