It’s not just Amtrak. On Eurostar, running from London to Paris and Brussels, they’ve quietly changed the rules.
It now costs a child’s seat to carry a cello on board, or 40 Euros if you send it in the luggage van.
Here’s the latest small print (thanks to Callum Thomson for the link, and the RLPO for the pic).
As for doubles-basses, in Spain they can refuse altogether to let them on board (link sent by Dianne Winsor). Read here.