It is the Silver Siesta, and, as Mark5 says, the original name is barely visible. This wasn't unusual where Amtrak removed lettering from stainless steel cars. I remember seeing it on a lot of cars in the early 70s when Amtrak was new.
Amtrak had quite a few Slumbercoaches, including most, if not all, of the NP's. Yes, Amtrak bought the Loch Ness. Whether it came with a monster is unknown.