An Amtrak bike BOX measures 69 1/2 long, 39 3/4 high and 8 3/4 wide. It's quite large enough for almost any bike if you just turn the handlebars and remove the pedals.

When we traveled with a pair of LWB recumbent bikes (each with the same wheelbase as a standard tandem), I made the BOXes even larger. Amtrak had no problem with me extending the BOXes by taping on 1/2 of a second BOX on each. The LWB fit in with the wheels still on, but the front fork turned around.