Despite everyone’s gripes that it isn’t roll on-off, taking a bike on amtrak from DC is not that bad. Just make sure to show up plenty early (at least an hour) at the station in DC or they’ll get stressed out. The boxes they sell (or give away) are plenty big for any bike with the handlebars turned and pedals takin off.
On amtrak, you can bring your own box. I think the price of their box is about $15 bucks if you need one of theirs and maybe another $10-$15 for the bike. I did this last year. The boxing is very basic, just remove pedals, handlebar, take off the front wheel and jack down the seat.
It was a great experience riding the train. They even bumped my ticket up 2 days early for free because I wanted to leave early. You just need to verify that both stops (boarding and departing) have checked baggage service, you can call ahead to verify.
Sweet deal!, I once tried to box-up a bike on the Amtrak going from Philly to Pittsburgh. They said they had to put it on a separate train with a baggage car. Trouble was- as I found later- that train was not actually going to Pittsburgh. And hilarity ensued!
I just came back from a GAPCANDO trip with 2 others and we rented a large SUV (~$300). We just fit the bikes in the back. Turned out to be about cheaper than each of us buying train tickets with bike storage. Faster trip back too.