Unfortunately, private companies own most of the rails so unless they decide to upgrade them, that's as fast as we can go. A lot of other countries with successful rail infrastructure has the government own them.

Taking a train vs plane can cut your CO2 emission quite a bit: https://www.seat61.com/CO2flights.htm

What are the regulations on staff? I'm not aware of what Amtrak is required, currently. I'd imagine that's their biggest cost, not weight. Although, I do like the idea of having a more compact/denser sleeping cars. The Roomettes are really small, but they come with attendants, etc.

Amtrak is currently upgrading some of their long distance cars: https://www.travelandleisure.com/amtrak-upgrades-overnight-t...