Some of the 15 derailed carriages were left stacked on top of each other, as rescue teams worked to free people trapped inside the train in Uttar Pradesh state, 150 kilometres (95 miles) south of Lucknow city.
"At least 35 people are dead and 100 injured are being treated at the scene and in hospital," K.N. Joshi, the local district chief medical officer, told AFP. "I have seen a number of people still lying inside the coaches."
According to a news agency, the driver was among the injured and that local people had rushed to the crash site to help pull victims from the wreckage of twisted metal.
The train, carrying about 1,000 people, was moving at near its top speed of 108 kilometres an hour when it derailed, PTI quoted one regional railway manager as saying.
The Kalka Mail train was travelling from Howrah, the main station for the eastern city of Kolkata, across India to the capital New Delhi when it left the tracks near the town of Fatehpur.
"We were sitting in our seats when suddenly everything turned upside down," said a male passenger interviewed by the CNN-IBN news channel. "When the train stopped we broke the glass windows to jump out on the track."
At nightfall, emergency teams were still working to cut their way into two carriages to search for survivors. Officials said the death toll could rise and that an investigation had been launched.
Prime Minister Manmohan Singh "expressed deep sorrow and shock at the loss of lives" and promised all resources in the area would be deployed for rescue and relief operations, his office said in a statement.
Anxious relatives and friends of the passengers gathered at Howrah and other stations along the line seeking information about their loved ones.
A special train was scheduled to take families of victims to the accident site, state officials said, while two military helicopters were also sent to assist.
Only three days ago, 38 people were killed in another rail crash in Uttar Pradesh when a train slammed into a bus carrying a wedding party.
India's state-run railway system — still the main form of long-distance travel despite fierce competition from new private airlines — carries 18.5 million people daily.
The worst accident in India was in 1981 when a train plunged into a river in Bihar, killing an estimated 800 people.
The railway is the country's largest employer with 1.4 million people on its payroll and it runs 11,000 trains a day.
Experts say the creaking system, the world's second largest under a single management, is desperately in need of new investment to improve safety and help end transportation bottlenecks that threaten the country's economic growth.
