The Shramjeevi Express train was travelling in the afternoon between Jaunpur and Delhi when, at 5:15pm, an explosion tore through one of the carriages.
As other passengers and locals aided those wounded by the blast, emergency services fought to extinguish the burning carriage.
RDX is a military grade explosive which had been used in several terrorist attacks on Indian targets, including the Ayodhya train bombing in June 2000.
Eyewitnesses reported two young men having boarded the train at Jaunpur with a white suitcase.
Officials deemed it highly unlikely that this was an accidental explosion and authorities attributed it to Islamist extremists.