A conductor was sexually assaulted on a train at Lincoln railway station.
A man grabbed the female conductor's bottom and squeezed it when she was carrying out a ticket check.
The incident happened on January 25 on the 6.24pm Lincoln to Sheffield service before it left the station.
And British Transport Police officers are now appealing for anyone who saw the incident to come forward.
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 pending further enquiries.
“We believe that the train was busy at the time of the incident and think a number of people may have witnessed this assault.
“If you were on board the service at the time, we urge you to come forward as you may have information crucial to our investigation.
“We take assaults against rail staff extremely seriously.
"The victim was simply doing her job and shouldn’t have been subjected to this unpleasant violation."
Anyone with information should call the British Transport Police on 0800 40 50 40 quoting background reference NEA/B5 of 29/01/2014.