The blast, which went off at around 2:00 a.m. on Thursday (8:30 p.m. Wednesday GMT), occurred on a train heading from the breakaway northeastern Indian state of Assam to the country's capital, New Delhi.
Four people were also hospitalized, while authorities say the death toll might rise.
The explosive device went off on the super-modern Rajdhani Express train soon after its departure from the Naujan station in Assam.
The Adivasi National Liberation Army (ALNA), a regional ethnic militant group, has claimed responsibility for the attack.
The ALNA operates mainly in eastern Assam, and is seeking special minority status for Assam's ethnic Adivasis.
The train continued on its way four hours after the explosion.
Terrorists made an attempt in January to derail the Rajdhani Express, but their bomb went off after the train had passed.
