I've travelled on Amtrak with my manual w.chair, and it should be no different with a power chair, as AfterMyNap/Cindy stated.

Ideally, there will be a working lift, as she also mentioned, but... it's not something I would count on. The lifts were not functioning when I travelled from Sacramento to Bakersfield (a couple or so years ago) and the porter asked me to help lift my wheelchair. Ehhh, what a twit-wit; if I could lift the darn'd thing I probably wouldn't be needing it in the first place. (He was definitely moron maximus.)

The menu food can be quite price-y; I took my own (from a deli) packed in ice. There is generally a charge for in-seat delivery of food, but... being disabled, no charge should be affixed.

I took my laptop and DVD movies; there are hook-ups in the car(s).

If you have any probs., don't hesitate to call on a car attendant. I found them to be quite friendly and helpful.

Will you be in a sleeper?

Amtrak also discounts those for disabled and automatically bumps the size of the room (no x-tra cost added); it is, however, first come, first serve basis (with reservations).

I also found it easier to take my non-carry-on luggage to the station for check-in a day early so that I didn't have to mess with it; it arrived at my destination and was awaiting my arrival .

P.S.: I don't know the travel route you're taking, and I'm not asking you to post that info., but, if there is a bus-transfer point, make sure that all modes of transportation involved know well in-advance that you will be travelling with your power chair and might need assistance.