Prosecutors on Tuesday opened a terrorism investigation into an explosion that threw an express train on one of Russia's major passenger routes off the tracks, injuring 60 people.

The blast about 9:30 p.m. Monday hit the Neva Express train that was en route from Moscow to St. Petersburg. The locomotive and a dozen passenger cars derailed near the city of Novgorod, about 300 miles north of the capital.

A duty officer for the national prosecutor-general's office said a terrorism investigation had been opened, but could not provide further details.

The Interfax news agency cited top regional prosecutor Sergei Bednichenko as saying that the blast had the equivalent power of about 4.5 pounds of TNT.

At least 60 people were injured in the derailment, 25 of whom were hospitalized, news reports said.

The train's driver heard a loud report underneath the locomotive immediately before the incident happened, Alexander Pirkov, an adviser to Russian Railways' top executive, said in televised comments.

Pirkov said 215 passengers and 20 crew members were on the train at the time. All traffic along the railway — a major passenger and cargo route — was halted.

One passenger told the NTV television channel by telephone that some people appeared to have been trapped in the cars, with conductors, train workers and other passengers smashing windows to try and help people as downed power lines crackled overhead.

The Moscow-St. Petersburg route is one of the busiest routes for the Russian Railways network, and is popular particularly among foreign tourists.

The blast and derailment tore up about a half a mile of track. Workers were laboring Tuesday to fix the rails, with the aim of restoring service by Tuesday afternoon.