By Subir Bhaumik
BBC News, Calcutta
Trains in the region have been targeted by insurgents in the past
A bomb has exploded in a passenger train in India's north-eastern state of Assam, killing at least three people.
Assam police chief GM Srivastava told the BBC that 30 other passengers had been injured, some of them seriously.
The train was travelling from Lumding railway station in Lower Assam to Tinsukia in Upper Assam.
The blast took place when the train halted at the Diphu station in the hill district of Karbi Anglong for a scheduled stop.
Assam has a number of insurgent groups which have a history of attacking the railways as they provide an easy target.
Mr Srivastava said the death toll could rise as the condition of some of those injured was "pretty bad".
The injured have been taken to a hospital in Diphu, but those with serious wounds are being brought to the state's capital, Guwahati.
Train services between Upper and Lower Assam have been disrupted but railway officials say they will restored within a day.
None of the many rebel groups active in Assam has claimed responsibility for the explosion so far.
But police say they suspect the involvement of the Karbi Longri National Liberation Front (KLNLF), a group fighting for an independent homeland for the Karbi tribes in Assam.
This group has been recently involved in several attacks on Hindi-speaking migrants in the district of Karbi Anglong and one such attack was reported on Tuesday from the village of Dolamara in which two Hindi speakers were killed.
A series of nine explosions in four towns in Assam on 30 October killed more than 80 people.
The state government blamed the separatist United Liberation Front of Assam (Ulfa) and the National Democratic front of Bodoland (NDFB) for the explosions.
In the past few months, there has been a series of blasts across many Indian cities including Delhi, Ahmedabad, Bangalore, Jaipur and Hyderabad.
But unlike in the north-east, Islamic militants have been held responsible for the explosions in mainland Indian cities.