Comment by nine_k

NYC tunnels have outsized cost, but even tunnels that cost 10% of that would not be cheap. Most of the NYC subway was built quickly and relatively cheaply by digging trenches and covering them later, not by boring tunnels. NYC used to use elevated trains a lot, and still has plenty of those outside Manhattan, but they have other downsides, such as noise.

NYC subway has two drivers per train due to the trade union resistance. Very understandable, even though not really necessary.

Self-driving buses could be also remotely driven in situations that require special handling, by a smaller number of drivers.

As usual, it mostly takes a political will, the technology is not rocket science; one of the poster kids of that is Curitiba in Brazil.