Some 12 carriages of the Rajdhani Express, which was travelling from the capital New Delhi to the northeastern state of Assam, toppled over at around 2:00am local time in Bihar state's Saran district.
"Prima facie, it appears to be a case of sabotage," Railway Board chairman Arunendra Kumar told the Press Trust of India (PTI) news agency. "There was a blast on the track, which could have caused the derailment."
The chief spokesperson of railways Anil Kumar Saxena said an expert team had rushed to the site to gather evidence.
"One of the possibilities could be sabotage by the Maoists," he told AFP.
Home Minister Rajnath Singh said it was "too early" to blame the insurgents, according to PTI, while a senior police officer said that a "technical fault" could have been to blame.
But Maoist rebels had earlier called for a day-long strike in the area to protest against the security forces and police said they had recovered three crude explosive devices that had been planted in a marketplace in a nearby town.
If confirmed as a Maoist attack, it would be the first by the insurgents since Prime Minister Narendra Modi's right-wing Bharatiya Janata Party came to power last month after a landslide election victory.
Separatist insurgencies affect large swathes of India's northeast, northwest and central regions, including the states of Chhattisgarh, Orissa, Bihar, Jharkhand and Maharashtra.
Many of the rebels say they are fighting authorities for land, jobs and other rights for poor tribal groups.
The Maoist insurgency has cost thousands of lives.
Eight other passengers were taken to hospital for treatment for their injuries following Wednesday's derailment.
One of the casualties, Ram Pratap Singh, described how passengers in sleeper carriages had been woken by a "loud noise".
"The carriage tipped over and over again and a passenger on a top (berth) fell on me," Singh told the ABP network.
"Then we got up somehow and found a small opening to get out," he told the channel.
Stranded passengers were being ferried in special trains while railway workers were trying to fix the long stretch of broken tracks.
It is the latest deadly incident on India's dilapidated rail network, which is still the main form of long-distance travel for the middle-class and the poor.
Last month, 26 people were killed when a passenger express travelling in the northern state of Uttar Pradesh ploughed into a stationary freight train.
In 2012 a government report said almost 15,000 people were killed every year on the network, describing the deaths as an annual "massacre" due mainly to poor safety standards.
Modi's government has pledged to revive the railway network by infusing funds and introducing new, modern trains.
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@pity Maoists are not separatist movement. They do not wish separate country. They are anti currupt politicians exploiting their resources. As far as others gangs of five people don't represpresent the states. See the voting ratio of north east all above 70,75%. Highest even from city like mumbai delhi. The only problem or worry we have is supported terror movement in kashmir.
Oh and Indians claim that there is no separatist movement going on India. People who have glass houses should not throw stones at others. ET Please publish.