The railway official, speaking on condition of anonymity because he was not authorized to speak to the media, said 13 passenger cars on a train out of Calcutta were derailed by the explosion, in a rural area about 90 miles (150 kilometers) south of the city. At least some of those coaches were then hit by a cargo train coming from the other direction.
Home Secretary G.K. Pillai told NDTV television that at least 15 people had died and at least 150 more had been injured.
The blast occurred in an isolated, rural stronghold of India's Maoist rebels, known as Naxalites, who have stepped up their attacks in recent months. However, officials cautioned that it was too early to say whether the blast had been triggered by the Maoists.
The passenger train was heading to the Indian city of Kurla.