Trains were last used to move nuclear WHs/Bombs in the mid-1980's because of security concerns. Deliberate acts (sabotage, etc) were/are more of an issue than accidents.

the US has been moving nuclear WHs via trucks for a few decades now....not exactly breaking news.

Perhaps it's too small of quantity to be of use but why not recycle the PU-238 from the RTG's used in nuclear weapons? We've retired/dismantled several thousands of nukes that used RTG's. The RTG production is now under the Sandia National Labs & they should know whether these RTGs could be of any use to NASA's problem.