Shrikant, who had an affair with Seema(24), was annoyed with the fact that the girl's family had fixed her marriage to a suitor of their choice, Kanpur SSP Yashashwi Yadav said.
The incident took place last night when the accused attacked the victim with some sharp-edged weapon while she had gone outside to attend to nature's call.
The girl died on the spot.
Later, Shrikant jumped before a moving train and committed suicide, Yadav said.
The mutilated body of the boy was recovered near the railway tracks in Nazirabad area this morning, the SSP said, adding that further probes are on into the case.