I take amtrak all the time to DC, Ive taken it to/prom Pittsburgh a few times too, and Boston once.
I mainly take Acela/metroliner, but have takenb regional once to DC, and took it to Pittsburgh. What youll find, is that on the northeast corridor (the straight trip from DC to Boston), it is pricey, but very fast. I can go from wilmington, DE to DC in about 1hr 15min... thats at least an hour faster than if I drove, and typically more like 1hr15 to 1hr 30 faster.
Now, going to Pittsburgh is another story. Amtrak only has right of way on a few selected railroads, like on the NE corridor. Everytwhere else (I suppose everywhere they use diesel engines rather than electric), they are on Norfolk Southern, CSX, etc. lines. This slows things down a lot. A drive to Pittsburgh would be about 5-5.5 hours, on a triain is 7.75 hours or so.
So youll be the same as a car or somewhat slower than a car, but you can sit back and relax, do work, etc.
Food isnt generally great - microwave-heated junk food and pricey sandwiches mainly. Bring what food you can - you will be able to take more or less whatever you want on the train (within reason, volume-wise), so I suggest that.
Unless you switch trains, youll be on it the whole way. Some stations they stop and people can get off and then right back on after a smoke break, but its not enough to see anything real. Is your train direct, or how does it go??? Also, beware of busy interchanges, where lots of people get on. When the train is packed, though youll have plenty of legroom, its not much different than a full airplane flight. Id suggest you sit in the aisle seat (theyre usually 2x2 across, like a small airplane, but with more room all around), and then put a backpack, etc on the window seat, so that it looks taken - youll be left alone better that way. Many folks pretend theyre asleep at station stops, just so people don't try to take the seat next to them.
Have fun and relax... taking the train is really an enjoyable way to go! I recall that Savannah is a nice town - I was there a few years back training for rowing.
JMH