An explosion on the railway near St. Petersburg injured a worker early on Tuesday morning and caused delays in the rail service.
A bomb, thought to contain 200 to 400 grams of TNT, went off just after 4 a.m. on the main line between Moscow and St. Petersburg.
The incident comes in the wake of a Nov. 27 bomb hit the Nevsky Express on the same route. In that attack, 28 people died and more than 90 were injured.
On Tuesday, the device went off under the wheels of a railway carriage, injuring a guard's leg, police said. The man was hospitalised.
Police are investigating what they suspect to be a terrorist attack.