Resurrecting this in case there is any new info. Seems like so far only the OP has encountered smaller boxes. I'm taking a short trip that involves Amtrak, and I measured my bike. Seems like the stated dimensions (
69 x 41 x 8.5 inches) will be cutting it close for me. Uncut steerer tube makes that the non-negotiable high point on my bike, close to the height limit assuming I remove the stem rather than simply turn the bars. Wide bottom bracket has me concerned about needing to remove my cranks rather than my pedals. And the length seemed the most problematic. With the rear fender, it's a very close thing. It's difficult to imagine a smaller box working for me.