Something I found online:

"Hello Amtrak here."
"Hi. May I purchase a child seat for my cello?"
"Is the cello over 15 years of age?"
"No. It's only 4."
"Well...you should be fine, then. Does the cello have a name?"
"It's Llew."
"That's really good. I'm trying to figure out what to put here."
"Llew Chelo?"
"No...I want them to be able to realize that it is a cello. We'll put __ Cello."
"Wow! Thank you so much!"
"Just make sure Llew Behaves."
"Of course."

"I asume if the cello is over 15, then it's an adult fare?"

"And antiques get a senior discount then?"

Hahaha :) Looks like I will also be Amtrak-ing (an overnight one) back to VA with the cello! Except I didn't buy a seat for it.