The used fuel rods have all now been removed from the reactors. That only takes a couple of years, or less. However, these were Magnox reactors, so have large amounts of graphite in the core, some of which will have been transformed into carbon 14, which is radioactive.
I don't know how they deal with that. They use robots for the decommissioning work, so maybe they extract the individual graphite blocks and take them away. C14 is a beta emitter, a simple metal box will contain the radiation. But if the reactor has ever had a ruptured fuel element, there will be lots of very nasty fission products in the graphite.
The pressure vessel will also be radioactive, and will be cut up into pieces and removed.
It does make sense to use rail for all of that, as it is safer than road.
Unfortunately I work on reactor control systems, not decommissioning, so I don't have detailed knowledge of what they do with all the waste. But what is sad is that only about 2% of the uranium fuel is actually used up, the rest is presently stored, at vast expense, when it could be reprocessed, and fed back into other designs of reactor which would consume many of the troublesome fission products. I hope that the new build of reactors which is coming soon will address this issue properly.
I wonder what the locals would feel about Trawsfynydd being the site of one of the new power stations? It probably won't be, as it is not owned by British Energy (or is that French Energy now?), my guess is Hartlepool, Sizewell, Hinkley Point and Dungeness will be the four new sites. But the locals near Chapelcross want a new power station, and the Scottish Parliament will not let them have one.
But I wonder why Trawsfynydd was chosen in the first place, as all the other sites are on the coast.
Alan