Yea, it's a rail cannon. But I can guarantee you it will not move on a commercial railroad in this country.
1. Never say Never. The military, if it so chooses, can cause bends to normal regulations although I agree they will look at other options seriously before they choose to have discussions with the FRA. Unless you wish to tell us that you are involved in the museum's business, you can not make definitive statements.
T rail canon will be taken apart to be moved
2. Remeber, this is a historic piece. There may be able to break it down at a couple of key points that may reduce overall weight and length(e.g. Tube from Lift Mechanism, Trucks from Frame, etc.) but they will not be able to alter it (read that as torch it) just to get into "manageable pieces". This is a 65+ year old HISTORIC piece, that only two exist in the world today. Every nut & bolt & gear is probably fused together by corrosion. If anything, they may have to re-inforce it for the move to keep it from shaking apart.
3. Finally, and someone can correct me if they know differently, I believe the unit was brought in by rail after WWII to APG for analysis and then later, display.
In the end, you may be right, that they can figure how to reduce the load and ship by truck, but any way you look at it, it will present quite a challenge.