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.