I showed up at the Shiphol AMS airport in NL and bought one there, KLM.

Amtrak requires boxed Bikes , so they have a Box to sell you, but you gotta go get it.

they have free boxes at most bike shops ,they are smaller.
so you have to take more stuff off the bike to make it fit.

buy a lot of packing tape to make 2 boxes into a long one if needed.