How else do you ship it? Radioactive materials count as pretty much anything irradiated, so for a Navy nuclear plant (something they transport on trains that is radioactive) that's every pipe, valve, and metal fitting from the primary loop. You're not shipping all that by truck or plane with more security than a train, which is pretty unstoppable and it's easy to anti-air missiles and stuff to a train vs. a truck. Also, you'd need dozens of trucks or planes for one reactor decommissioning vs. one fairly compact train.