Recently had the opportunity to ride the City Circle Line or M3 in Copenhagen Denmark. I was expecting a generic urban subway, with platforms and trains rushing by. Instead it more resembled an airport people mover. Doors keep people away from the tracks and open when the train arrives. It was fast and quiet and it was all automated. Not even any drivers. About ten miles long with 17 stops finished in 2019 for $3.4b. I can think of a few cities in the US that could use something like this.

City Circle Line