Part of the issue with the separation between casks / cars is that you have to make sure that in the event of an accident, you have only the low level of radioactive decay emitted by the waste. If enough waste were to be combined (say, with several cars falling down an embankment and ending up on top of each other), you would be at risk for a criticality incident, where there is enough mass present to sustain a nuclear chain reaction... this would be VERY bad (and happened a couple of times during our early experimentation with nuclear energy) -- bombs achieve this criticality by forcing two sub-critical masses together abruptly, reactors by gradually withdrawing control rods and allowing an already critical mass to achieve the chain reaction. You probably would not have an explosion -- fuel is not nearly as pure as the weapons grade stuff -- but it would generate a lot of heat and radiation, which would be deadly for a large radius around the accident.