Rampur's Superintendent of Police KK Chaudhri, who visited the accident site, said the track was found broken and some of its portion buried.
"Sabotage cannot be ruled out," he told PTI when asked about possible reasons behind the accident.
A railway source in New Delhi also said the derailment was caused, prima facie, due to the rail fracture.
"Weld was found broken. But the reason of the fracture would be ascertained through investigation only," the source said.
Mr Chaudhri said an investigation has been initiated and entire fact would come out only after its completion.
The accident occurred two kilometers from Rampur junction and about 100 meters from the Kosi river bridge.
While Rampur police said 15 passengers were injured, three of them seriously, the railways have maintained that only two passengers - Amit Katiyar and Megh Singh - have received injuries.
According to the police, seven passengers have been discharged from the district hospital after first aid treatment. The three seriously injured passengers are out of danger, the SP said.
Several senior railway and police officers are present on the site.