Russian officials say a train operator has suffered a leg injury after a bomb exploded under a railway technical inspectors engine in St. Petersburg.
The engine, used to inspect and repair line side equipment, was running without any carriages.
Authorities have started investigating the Tuesdays blast, which occurred at 14:15 local time, as a terrorist attack.
We consider it a terrorist act, thats the main theory, Interfax news agency reported, quoting Anatoly Kvashnin, the head of a regional investigative department for transport systems.
Kvashnin did not elaborate on whether authorities were looking at any possible suspects.
The latest incident may reignite fears of attacks on Russias railways.
Earlier this year, RIA Novosti news agency reported that an explosion had ripped through a cargo train in the Russian republic of Ingushetia, leaving no injuries.
The deadliest attack, however, occurred in late November, when a bomb killed 26 commuters on a train en route to St. Petersburg from Moscow.
Days later, another blast hit a passenger train in the troubled southern Republic of Daghestan, but no injuries were reported.