So if you look at the route from the airport to downtown and/or midtown (Vanderbilt area) you cross I24, I 440, I65. I created a map from BNA to W Nashville to help clarify.
I40 is similar to I35 in that those are to 2 most traveled Interstates in the US for heavy (trucking) traffic.
I65 going north south is also an extremely busy Interstate to Birmingham to the south and Louisville to the north.
I24 is traffic for north and south bound traffic for Atlanta. I would say there is an accident or car stopped between 7-9 then 3-7 about 20-30% of the time I go through (state workers get off at 3 pm so the traffic hours are longer than most cities in the afternoon).
The civil engineer that designed the Interstate loops around Nashville clearly failed fluid dynamics.
I used to have a place in Hermitage just east of the airport. I ALWAYS budgeted 30 minutes to get downtown.
Nashville, like Austin is a massive tourist city. #1 Bachelorette party place in the US (or was yay!!), with an NFL AND NHL team downtown. I DON'T MEAN CLOSE TO DOWNTOWN. Then there are the Country Music Award and all sorts of similar stuff to big Austin festivals.
This under ground train would be a pretty similar trip, with MORE traffic congestion issues than Bergstrom to downtown Austin than BNA to downtown Nashville. The worst, like Austin is big weekends on Thursday/Friday then Sunday/Monday.
Now, tunneling in TN is.... um.. tricky. A train is absolutely useful and helpful.