Russian prosecutors launched a terrorism investigation on Tuesday after an improvised bomb derailed an overnight express train, overturning carriages and injuring dozens of passengers. "The train accident was caused by a homemade explosive device," Sergei Bednichenko, chief prosecutor for Russia's North West district, told Channel One television. "A criminal case has been opened under article 205, clause 3, that is terrorism." There was no indication of who may have planted the bomb, Reuters reported. The derailed train was an express service traveling between Moscow and Russia's second city of St. Petersburg -- one of the country's busiest rail routes. Sixty passengers and train crew were injured in the derailment, of which 38 were admitted to hospital, a spokeswoman for rail operator Russian Railways said. About 250 people were on board in all. The incident happened late on Monday near the village of Malaya Vishera in the Novgorod region, about 500 km (300 miles) north of Moscow and 170 km from St. Petersburg.