A train conductor from Sheffield was sexually assaulted by a passenger as she was checking tickets.
The 40-year-old was working on a train travelling between Lincoln and Sheffield when a man assaulted her last Saturday.
Officers investigating the incident want to hear from witnesses travelling on the 6.24pm Lincoln to Sheffield service.
PC Ewan Brechin, leading the investigation, said: “The train conductor, a 40-year-old woman from Sheffield, was conducting a ticket check on the train when a man gabbed her bottom and squeezed it.
“She remonstrated with the man before she left the carriage.
“The train was met by police at Sheffield station and the man, aged 29 and from the Sheffield area, was arrested. He had been released on police bail.”