Railway spokesman Anil Kumar Saxena said the fire broke out about 3:15 a.m. local time when the Bangalore-Nanded express was traveling through Andhra Pradesh state, CNN reported.
About 60 people were aboard the train at the time.
Many people pulled the safety chain and jumped off the train, the Times of India reported.
Those killed were so badly burned they could not be immediately identified. Railway officials said reservation charts were being used to help in identification, but DNA testing may be required.
The cause of the fire was not immediately clear, but a short circuit in air conditioning in the compartment was suspected, the report said.