An explosion derailed several cars on a Russian passenger train traveling from Moscow to St. Petersburg late Monday, an official with the national railroad operator said. An emergency official said at least 27 people were injured, three of them critically. The train's driver heard a loud bang from underneath the locomotive immediately before the incident happened, Alexander Pirkov, an adviser to Russian Railways' top executive, said in televised comments. The derailment occurred around 9:45 p.m. (1745 GMT) near the city of Novgorod, about 500 kilometers northwest of Moscow. The RIA-Novosti news agency also quoted several passengers as saying that they heard a loud bang immediately before the derailment. Investigators were trying to determine what caused the explosion, Pirkov said. Nine cars derailed in all and 215 passengers were on the train at the time. All train traffic along the railway - a major passenger and cargo route - was halted, he said.