The explosion occurred shortly after the train arrived at Diphu railroad station, about 200 miles (300 kilometers) south of Gauhati, the capital of Assam state, said District Magistrate M.C. Sahu.
The train was heading from Lumding in central Assam to the eastern commercial hub of Tinsukhia, Sahu said.
While no one immediately claimed responsibility for the attack, an intelligence official said an ethnic insurgent group, Karbi Longri National Liberation Front, fighting for wide autonomy in the state for the past five years was suspected. He spoke on condition of anonymity as he was not speak publicly about the details.
The front is one of the three groups active in the region; the other two groups have reached cease-fire accords with the government.
The blast comes just days after suspected Muslim militants attacked targets across Mumbai, killing at least 172 people and injuring 239. Tuesday's blast was not seen as related to the Mumbai attacks.
In October, the state had witnessed 13 coordinated bomb attacks which killed 77 people and wounded hundreds in four towns.
Several insurgents groups are battling for power, for ethnic pride and for control of drug routes in India's northeast, an isolated collection of seven states and hundreds of ethnic groups and subgroups. They fight the government and they fight each other in a region crippled by poverty and political chaos.