The train was travelling from Haldibari to New Jalpaiguri when the explosion hit one of the carriages at Belakoba station, 500 kilometres (310 miles) from the state capital Kolkata, said Prasad Ranjan Roy, the state's home secretary.
"Four people were killed on the spot," Trikal Rabha, a North-eastern Frontier Railway spokesman said from Guwahati city in neighbouring Assam state.
"The toll may go up as 25 people of the wounded are in a critical state," he told AFP.
Rescue workers were trying to free the injured from the mangled carriage but were being hampered by smoke and fire, Rabha said. The injured were being transported to hospitals in New Jalpaiguri, 18 kilometres (11 miles) from the blast site, he said.
"Six of the injured brought in to our hospital have died," said doctor J. Burman who is treating the injured in one of the hospitals. "We have admitted 51 people including nine children and 14 women. Some of them have multiple injuries and need emergency surgery," he said.
It was not immediately known what caused the explosion as the area where the blast occurred is in a remote corner of West Bengal state, bordering the Himalayan countries of Bhutan and Nepal, police said.
Separatist rebels of the Kamatapur Liberation Organisation (KLO) -- fighting for a separate homeland -- are known to be active in the region, police said.
The KLO is known to maintain close links with the banned United Liberation Front of Asom, one of the most powerful insurgent groups operating in India's restive north-east.

Copyright AFP (Agence France-Presse), 2006