After reading this forum for about a week, I have wanted to bring up San Francisco. I am glad it has now been brought up. I believe that is the model that Nashville should follow. Let me elaborate further. SF has 2 separate systems. BART is heavy rail that serves the bay area, it connects the Airport with downtown SF where it is underground and stations are close together. It then travels under the bay to Oakland and serves more as a commuter rail system with stations at much wider distances. There are 4 lines beginning in various cities north south and east that cross the bay with a fourth that only serves the Eastern part of the bay from north to south. Trains are less frequent on the outskirts and increase in frequency as lines converge. Within SF they share a single tunnel making the time between trains very small. I believe they are currently trying to expand the system south to the southern end of the bay.
The second system is the SF MUNI. It is LRT and is very similar to the Boston Green line and one of the lines in Philadelphia. As noted above it shares stations with BART in the most dense area of downtown. The different lines separate and spread southwest through the city, As with BART trains are much more frequent in the tunnels because they converge into a single line. throughout the rest of the city the trains run on tracks embedded in streets. They have recently built a new line that is completely above ground and are building a new (controversial) line that runs perpendicular to the existing lines.
Since the city is roughly 7 miles square, LRT works great with in the city, (the bus and trolley bus system are also outstanding)
Heavy Rail works great as a downtown subway and commuter rail.
While visiting last year I used both systems while sightseeing. Google maps on my phone was dead on with schedules and directions for getting across town and even crossing the bay to see the university of California in Berkeley and going to Alameda to see the USS Hornet.
To translate this to Nashville as a long term plan.
Build the BRT as planned on west end. With future lines on Charlotte and even into Germantown and 12th. Some eventually to be Replaced with LRT. Build a heavy rail system to Franklin, Murfreesboro, and Gallatin all following existing RR row.
These can initially arrive at a main downtown station near the old Union Station. This could also be a connection for the return of Amtrak to Nashville.
Eventually they should be moved into a tunnel under downtown either under 4th or 5th ave.
I read one report that building a commuter line to Clarksville like the existing Music City Star already has sufficient demand based on existing park and ride buses.
There also needs to be something connecting the airport with downtown. It could be a bus connecting to a stop on the Murfreesboro HRT line, or a direct LRT to downtown.
Obviously, I realize I just listed a $Billion or so. But this could all be phased in over years, a single line at a time or a multiple lines at once but in short distances at a time. As previously stated we are behind Atlanta in the 70s. Buffalo was mentioned, but not its population, at the time they built their system they were the size of Nashville now, but the population has now decreased like most rust belt cities. In other words, Buffalo has a functional rapid transit system with a smaller population than Nashville.
The only gripe I had in SF was that the 2 systems had separate fare systems ( I believe they were in the process of changing that). I purchased a tourist pass for MUNI (good for LRT, Buses, trolley buses, the heritage street car and even the cable cars, great deal if you ever go) at the airport and simply flashed it to the driver when I got on a bus or at a window to ride the LRT, But I had to get a separate pass for BART.