Initially, there was some dispute over a berth in AC first class as the former MP boarded the train without a confirmed ticket, they said.
Singh, who is SP's official candidate for the next Lok Sabha elections, boarded the train from Pratapgarh and was on his way to New Delhi, they said.
Soon after his detention, the SP leader complained of pain in the chest after which he was admitted to the district hospital, police said.
The matter is being investigated by police and the victim has lodged a written complaint, they said.