Yeah, that's true, but underground rail is $4b/mi in San Francisco and a used Tesla Model 3 is $27k so you could get 5.4 used Teslas per San Francisco resident for the cost of one mile of rail. It's a cost issue.
Besides that, I did pretty fine with just an e-bike, in the last year that I lived in the Bay Area.
For Tesla to run you need roads, you need charging infrastructure, you need energy infrastructure and so on and so on. Not to mention that if you actually bought that many 'used' Model 3s the price would go up. Not to mention the huge amount of people who currently do not have a car, who all of a sudden would make the bad traffic (and everything associated with that) worse.
And guess what, the 'rail' would be 100x cheaper if some of the cars were moved of the road, and the rail was moved on that right of way. How about you do a cost comparison where the rail gets the existing right of way, and all those Tesla have to go into a tunnel. The way you phrase your whole proposition is already incredibly loaded.
The cost are so high literally because the San Francisco people who propose this stuff are bunch of Tesla driving asshats who don't undrestand the first thing about public transport. They make projects incredibly expensive because (a) they don't give a shit about the plebs who have to use it and (b) the bigger the project the better the politics of handing out money to people.
By any estimation, for a society as a whole having an efficient public transport system is by far a win. Everybody who has actually done any real analysis of the 'lets buy everybody a car' 'solution' will come to the conclusion that its incredibly dumb. Its just something far right wing people bring up to continue to prevent actual progress.