I crossed paths with an Xtracycle employee on a tour, and asked him about this (I was wondering myself).

He said they -- the people at company headquarters in NSJ -- have done it quite a bit, in traveling between NSJ and SF. He picked up the bike (there is a grip/balance point that he went to on the frame) and demonstrated. 'No harder than with a conventional bike, really' was the gist of the demonstration. 'Just carry it onto the train like this.'

Those Amtrak cars (the ones on the route that they frequented) have bike racks.

He said the racks worked, as I recall (though there may be some kind of trick to fitting the Xtra and getting them to stay [it's been a little while, and I don't remember that part in detail] -- but it was basically quite doable and easy).

Some Amtrak cars let you put your bike (unpacked -- just carrying it on and wheeling it around inside) in places other than the racks (especially if the racks are full). I've put fully loaded touring bikes in those racks; and I've seen people wheel their bikes inside and park them next to their seats, on the lower level.

*****
Some trains and employees are more bike friendly and bike-ready than others. Some are more casual and laissez-faire than others.