We decided to travel around the US using AMTRAK for a number of reasons: firstly, our terrible driving; secondly, the romantic idea of covering huge swathes of land by train; thirdly, it was a cheap way to see a lot. We thought it would also be totally flexible.
To some extent, these ideas were fulfilled.
The first thing to note about AMTRAK is that it's a lot cheaper if you're not a US citizen. Then you can take advantage of a single monthly ticket that covers the entire country. This isn't available to US citizens, so I can't say if it works out to be cheap for you if you are American.
The second thing is that AMTRAK trains are slow and old. There is a kind of pleasure in this: they were all built in the '70s, and AMTRAK, a company in trouble in car-hungry America, hasn't updated them. They are sleek and silver and very, very '70s. They are not full of modern amenities, but you get "old-fashioned service." They are, however, very comfortable. We traveled in coach the whole way, but you get more room than first class on an airplane. Of course, you are on there for longer.
That said, there are some essential things to take with you to ensure your comfort:
1) A sleeping bag, or some kind of blanket. Actually, most American travelers we saw had these "bed in a bag" things you can get at Rite-Aid in the US. Something that will keep you warm: AMTRAK trains get cold at night.
2) Food. Especially if you are vegetarian. The plus side to eating in the dining car is the social aspect. When we did this, we got seated with other travelers. We were lucky enough to meet some nice, interesting people this way. If you take it in your stride, this can add considerably to your traveling experience. The food is appalling, but edible at a pinch. They do carry vegetarian options, but they are not the most inventive and delicious cuisine you will sample on your trip.
3) Something to do. Those trips are long, and remember there's nothing to look at when it gets dark. We took a laptop, so we could watch a few movies, but only some of the routes have a plug by each seat (we only found this on the Washington-to-New Orleans stretch). Otherwise, you'll find one or two in an inconvenient place in the car.
The stations you will end up in vary greatly, but are characterized by their impracticality. There are good points. Denver station, with its lovely, friendly diner and faded glory, is one.
Don't expect to be able to buy anything practical at any station. Don't expect to be using Grand Central or anything like it. Both platforms and stations tend towards the ugly side.
Tickets. Although you may have bought your rail pass, this isn’t the end of it. In order to secure a place on the train, you need to reserve seats on specific trains. You need to book by phone and then collect the tickets at a station before you go in a spectacularly inefficient system.
This doesn't reserve you an allocated seat. This is done on the train. Depending on when you get there and how full the train is, you may or may not end up sitting with the people you are with. The staff do go to every effort to do this, though, and we were sat together on every leg of the trip.
The arrangements are different at every station. All part of the charm of the AMTRAK way!
AMTRAK is a slow means of transport. The trains travel on lines used for shipping goods across the country, and they have a low priority. The companies that use the lines for other things want to shut AMTRAK down because they slow them down, so this may be an experience you won't have for long. Expect long delays as standard. Expect to go slower than your average city commuter train.