Mayor Megan Barry's transit plan will include a significant underground tunnel through downtown Nashville that would enable future light trail and electric buses to avoid the narrow streets of the central business district, according to multiple sources familiar with the proposal.

The tunnel would stretch north-south from Music City Central, the city's bus hub on Charlotte Avenue, past Broadway, where there would be a passenger loading point, to a location near Lafayette Street south of downtown. It would go underneath Fifth Avenue South.

The idea would seemingly be for future transit entering downtown from the south on corridors such as Murfreesboro Pike and Nolensville Pike to connect to the south end of the tunnel. Light rail along Gallatin Pike and Charlotte Avenue would enter the tunnel on the north end near Music City Central.

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The price tag for the underground portion of the plan is not known as yet, nor where it would fall on a 30-year timeline for the overall transit project.

Barry is not formally unveiling her transit plan until Tuesday. When asked to confirm or deny the tunnel's inclusion in the plan, Sean Braisted, the mayor's press secretary, only pointed to Tuesday's meeting.

Barry outlined preliminary plans in April for a light rail system that would begin on Gallatin Pike in East Nashville. Her goal is to hold a public referendum in May to let voters decide whether to allocate dedicated funding over 30 years to pay for a potentially $6 billion regional transit project.

The mayor intends to outline a funding proposal at Tuesday's announcement in addition to more detailed designs. A telephone poll financed by her campaign committee last week tested residents' attitudes toward raising sales taxes, hotel taxes, car registration fees and business taxes to pay for mass transit. Barry has said to expect a combination of four tax revenue streams in her proposal, including sales tax.

Downtown tunnel has been on radar

A tunnel has increasingly been floated as a possibility for downtown. Nashville's downtown streets are decidedly narrower than other cities with light rail, posing engineering challenges to creating dedicated lanes for surface light rail. A tunnel would also reduce the amount of right-of-ways acquisitions needed for the project.

In an interview with The USA TODAY NETWORK - Tennessee last month, Barry said a tunnel is "definitely something that we’ve been working toward." At the time, she pointed to ongoing engineering work to address the downtown dilemma.

"A tunnel might solve a lot of those issues and also free up the street space to continue to have other types of modalities," Barry said, adding that her team was formulating a cost estimate.

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The Nashville Business Journal in August first reported on the idea of Metro officials exploring underground transit.

Nashville's hard limestone had long been seen as a non-starter for underground transit or a subway being a realistic option, but other cities facing similar issues have used new technology to build tunnels. The Business Journal cited a project in Seattle that has taken advantage of two new drilling techniques to navigate rocky terrain.

A Metro Transit Authority report released in August fleshed out how a five-corridor light rail network could work, what it might look like, and where Metro faces construction and right-of-way challenges. But the same report left a big question mark over downtown, where each of the corridors feeds.

Barry's presentation for her transit plan, dubbed "Let's Move Nashville: Metro's Transportation Solution," is set for 10:45 a.m. Tuesday at Music City Center.

Reach Joey Garrison at 615-259-8236, jgarrisno@tennessean.com and on Twitter @joeygarrison.