Although it is an interesting idea, it NEVER will happen. Nessman summed it up quite well. Having some experience in the nuclear field, I can add this; In the early days of nuclear power (1940's and 50's) in general there was more emphasis on beating the russians than there was on safety. ALCO got involved as a supplier because they were into heavy manufacturing and close to Knolls Atomic Power Lab (KAPL) wich was under DOE licence and owned by GE at the time for developing naval nuclear reactors. As stated in earlier posts, they were able to produce some specialized quality items like heat exchangers. There was, at the same time in Idaho, attempts to build a mobile reactor (via semi truck) for radar stations in Alaska and Canada (type SL-1) and a type for a bomber. The SL-1 was actually built, but it had a major accident after only a few years of operation and that program was scrapped. The aircraft reactors never got off the ground, but significant infrastrucure was built. (Getting back to railroading...) http://www.inl.gov/proving-the-principle/chapter_13.pdf
on page 8 as viewed in the PDF shows the highly SHIELDED loco and turntable used for moving the aircraft reactor around. If you look carefully, you will note that the track is std gauge for the loco and a very wide gauge for the car that carries the reactor.
If the loco were to be nuclear powered, there would be a need for shielding for the crew (front and rear) of the reactor and on the sides for the general public. If memory serves me, the limit for the general public is 1mr/hr. If the engine is moving, no issue, but if it stops anywhere there would be alot of issues with the several tons of lead and steel needed for this. NIMBYS would always say there is an issue.
The security would also be an issue. The cars that haul nuke waste (even before 9/11) typically have passanger cars/ caboose for the security detail that rides along. I looked for an old thread (couldnt find it - may have been in version 2 of the forum as it was prior to 2004) that dealt with the loading of a nuclear shipping container onto a flat car at Ballston Spa, NY 5-10 years ago which talked about security.
Water wouldnt be an issue as the plant itself would be leak tight and wouldnt need to be refilled in transit as a steam engine would. The water issue would be for emergency cooling should there be an accident (several tank cars and thus more weight).
The only way a nuclear engine would become viable for mobile use would be if direct conversion from nuclear energy (electrons, protons and such) to electric power become a reality. This not only would reduce the size and weight of the reactor, but would eliminate the steam plant (turbines and such) and need for water. This is not going to happen in the civilian world for decades (if at all) even though the military may or may not be working on it now http://www.kaplinc.com/whatwedo/index.html