The comparison here is to a transit system that uses vehicles which can be serviced by car mechanics. This will be cheaper.

And rail has otherwise struggled in the US.

The amtrak story is not great. I've tried to use them repeatedly, and outside of northeast corridor it is cheaper to fly then to take them anywhere. Let that sink in, the cost to get you up in air, and FLY is cheaper than rolling along on low friction rails. When you say why is it hard to understand that rail is cheapest it's because folks KNOW that things like buses are MUCH MUCH cheaper.

This is ignoring the absolutely stupid cost to build one mile of rail tunnel.

Building a tunnel and then filling it with cars is a waste of space. The per capita gains don't just come from being underground, they come from density of people - which is abysmal for cars, multiplied by the fact that most people drive alone for most of the time.

Mass transit rail has hardly "struggled" in the US - it has high usage and conflating the distinction between mass transit rail and long distance rail is disingenuous when considering the usecases of Musks tunnels.

Yes, rail transit is far more cost-effective than private automobiles when it comes to getting people from one place where they don't want to be to another place where they don't want to be.