Fourteen carriages of the Patna-Indore Express train went off the tracks in the early hours of Sunday near the Kanpur Dehat district, officials said.
"The incident, which took place around 3am, also left over 150 others injured," said IG (Kanpur range) Zaki Ahmed.
Some agencies reported the death toll has already gone up to 63.
Railways Ministry spokesperson Anil Saxena said several people with injuries had been rushed to nearby hospitals, and the number of casualties may rise. Accident relief trains have rushed to the spot, he said.
Northern Central Railways spokesperson Vijay Kumar said that doctors and senior railway officials had reached the spot and rescue and relief operations were being carried out by district administration and railway officials.
Though the cause of the derailment was not immediately known, sources said the nature and timing show that the accident was caused by a rail fracture. However, the exact reason would be known only after an inquiry.
Buses are pressed into service to help passengers complete their journey, Kumar said, adding that the S2 coach was severely damaged. Four AC coaches have also been derailed.
"All rescue relief work on to deal with unfortunate accident. All medical and other help rushed. Enquiry ordered. Situation monitored closely," Prabhu said in a tweet.
Another train accident in Uttar Pradesh in March last year killed 39 people and injured 150.
Prime Minister Narendra Modi expressed his condolences on Twitter.
"Anguished beyond words on the loss of lives due to the derailing of the Patna-Indore express. My thoughts are with the bereaved families," Modi said.