Has this been brought up yet?
Article from 2017
While the "Let's Move" proposal clearly has its genesis in the nMotion Plan, a 25-year plan for the entire Midstate unveiled last year, it goes farther, particularly with the proposal for the huge tunnel underneath the city. The billion-dollar, 1.8 mile tunnel would run from Music City Central at Charlotte and Fifth under Fifth Avenue South to Lafayette, where a second, Music City Central-style hub would be built. Passenger access would also be available as the tunnel goes below Lower Broad.
https://patch.com/tennessee/nashville/5-2b-nashville-transit-plan-calls-extensive-rail-huge-downtown-tunnel