British Transport Police (BTP) are appealing for information after a 17-year-old girl was allegedly sexually assaulted on a Lincoln to Sleaford train.
Officers are asking for passengers who spoke to the teenager to come forward.
The incident happened on board the 7.10pm Lincoln to Skegness service as it travelled between Lincoln and Sleaford on Friday, September 19.
The teenager reported that a man tried to kiss her, and then repeatedly exposed himself.
A 43-year-old man from Skegness was arrested at Sleaford station in connection with the incident.
He has since been released on police bail pending further enquiries until Tuesday, October 21.
PC Jackie Wadey, leading the investigation, said: “The victim, a 17-year-old girl from Lincoln, was travelling on the service when a man attempted to engage her in conversation.
“When she declined the man tried to kiss her and then exposed himself. The victim moved seats, only to be followed by the man, who continued to expose himself.
“Worried for her safety, she did the right thing and made others in the carriage aware of the man’s actions and phoned the police.
“The train was met at Sleaford and the man was arrested and taken into custody.
“A number of passengers on the service spoke to and supported the victim but left the scene before officers managed to obtain their details.
“If that was you please contact us as you may have key information which will assist in this investigation.
“We would particularly like to trace a red haired man with a bike who left the train at Ruskington, and an elderly couple who spoke to officers.
“Unwanted sexual behaviour of any kind will not be tolerated, the victim, who must have been terrified, showed great courage and strength in alerting other passengers and reporting the incident to the police.
“It is vital that we now speak to any witnesses on the train to gather evidence to prosecute and bring the person responsible to justice.”
Anyone with information is asked to contact British Transport Police on 0800 40 50 40, or text 61016, quoting reference MSUB/B12 of October 1.
Information can also be passed anonymously to the independent charity Crimestoppers on 0800 555 111.