Myself and two friends took the Empire Builder to Glacier National park back in August. Two of us had touring bikes that we put in Amtrak bike boxes my other friend had a trike that he put in a square (cube) box. His box still had to meet the size requirements and he was charged the same as a regular boxed bike. My guess is you will have to box the trailer and meet the size requirements. We had no problems with transporting the bikes/trike and the cost was very reasonable.