A bombing derailed a train traveling between Grozny, the capital of the restive Chechen republic, and Moscow on Sunday morning, injuring 15 people. The incident could have been far more devastating had the train not already been slowing to round a bend when the device was detonated, Russian officials said.

The explosion, which occurred about 90 miles south of Moscow, came as Russians were enjoying a long weekend marking the Russian parliament's declaration of sovereignty from the Soviet Union on June 12, 1990. Chechen separatists have staged terrorist attacks on Russian holidays in the past, but officials said they had no information on who organized this bombing. Prosecutors opened a criminal investigation on suspicion of terrorism and attempted murder.

A spokeswoman for the Federal Security Service, the domestic successor to the KGB, said a bomb containing the equivalent of about six pounds of TNT was detonated under the train's locomotive. Officials said they found wires attached to a rail and to a location about 50 yards from the tracks where the bomb might have been remotely triggered.

Six cars fell off the tracks, but none overturned. Four passengers, including an 18-month-old girl with second-degree burns, were hospitalized and about 40 other people sought medical care, the Russian news agency Interfax reported.

"The slow speed of the train and effective brakes have averted a far worse catastrophe," said Alexei Panteleyev, deputy governor of the Moscow region, according to Interfax. The Interior Ministry also announced that it had stepped up security on the country's railroad network, particularly on trains traveling to and from the south.

The Grozny-Moscow line opened last year after repairs were made to the central station in Grozny and to tracks, which were destroyed or damaged after war broke out in the republic in 1999. Russian officials had heralded the resumption of service on the 1,000-mile, two-day trip as a sign that life in Chechnya was normalizing after years of war. But the republic continues to be plagued by daily fighting between government forces and insurgents, and by abductions and terrorism.

A train derailed about 90 miles south of Moscow, injuring 15 people, after an explosive device was detonated.