At least eight passengers were injured when the engine and 11 coaches of the Rajdhani Express train toppled from the tracks near the Golden Ganj station near Chhapra town, said district administrator Kundan Kumar.
Officials said a freight train also went off the tracks in nearby Motihari town and about 18 cars were derailed. There were no casualties in the second incident, railway officials said.
Rajiv Pratap Rudy, a lawmaker from Chhapra, said there were about 500 passengers on the train that was traveling from the capital, New Delhi, to Dibrugarh in the remote northeastern state of Assam.
Railway authorities and police were investigating whether the first accident was caused by sabotage by Maoist rebels who are active in eastern and central India. The rebels had announced a general strike in Bihar on Wednesday.
"It's a very ghastly accident. It is very difficult to say whether it is sabotage or a derailment," Rudy said.
The rebels have been fighting the government for more than four decades, demanding land and jobs for tenant farmers and the poor.
In November, Maoist rebels boarded a passenger train and shot and killed three police guards while it was passing through their stronghold in eastern India.
The rebels have in the past blown up train tracks, attacked prisons to free their comrades and stolen weapons from police.
The Rajdhani Express is part of a network of fast trains connecting many Indian cities and is immensely popular.
Railway accidents are common in India, which has one of the world's largest train networks and serves 23 million passengers a day. Most of the accidents are blamed on poor maintenance and human error. AP