The Nevsky Express came off the tracks at 18.34 GMT, carrying 661 passengers from Moscow to St Petersburg.

Local officials put the death toll at 25, but the figure then rose to 39 after more bodies had been pulled from three wrecked carriages.

“There is objective evidence that a small crater had been found at the scene, sparking speculation of a blast from an explosive device is one of the explanations for incident,” Russian Railways chief Vladimir Yakunin told reporters at the scene.

Officials said that the toll could rise as 18 passengers were still unaccounted for.