This is questionably accurate.

It's not just a matter of building the rail, it's also redesigning the urban sprawl. That's a LOT of new construction of buildings needed, too. That comes with new utilities, etc. And cement is a huge carbon source.

There is a time scale over which that's more carbon efficient than replacing all personal vehicles and their replacement lifecycles, but it's very unclear if that's actually faster with regards to climate change timelines.