> Trains involve massive capital outlays

And self-driving cars don't? How many trains could be built with the billions invested so far in self-driving cars?

That's a one time cost. If we get the technology working building more self driving cars will be much cheaper than building new trains.

Self driving cars are simply a backwards-compatible transit solution. We have already invested so much money in car-based transportation to the detriment of trains. But, we will still need to continue paying billions and billions to repair and build new roads.

It costs NYC about 2.6 billion to construct a mile of subway.

In light of that, the money invested in self-driving cars is pretty small.

either that or NYC is just really inefficient in terms of cost.

Keep in mind you've cited probably the first or second most expensive mile of any subway ever built, and subways usually cost much more than other trains.

China regularly builds high speed rail for $30M per mile. Europe is usually closer to $50-60M. Tokyo has built subways for $400M per mile.

https://www.nytimes.com/2017/12/28/nyregion/new-york-subway-...