Plant operators Nuclear Electric described the incident as "very minor". But anti-nuclear campaigners said it underlined the dangers of transporting nuclear fuel by rail.
Spent fuel from the two Heysham advanced gas cooled reactors is transported to the Sellafield reprocessing plant in Cumbria every week. The empty flasks are then taken back to the Heysham site.
An investigation was launched by rail inspectors, Railtrack and the freight company EWS, who were transporting the flasks.
A Railtrack spokesman said: "At no time was there any danger to any member of the public. But we take any form of derailment very, very seriously and investigate it accordingly."
Emergency services were called to the scene. Special lifting gear had to be called in to hoist the locomotive's front axle back on to the track.
A Nuclear Electric spokeswoman said the 52-tonne forged-steel flasks had been tested to withstand 100mph collisions and the accident would have presented no danger even if the flasks had been full.Reuse content