A man has been arrested on suspicion of rape after a woman was allegedly attacked in a train toilet.
British Transport Police officers said the incident, reported on 8 December, is said to have taken place between Bristol Temple Meads and Swindon stations.
A man and a woman reportedly boarded the train at Bristol Temple Meads at 5.18pm and sat in a middle carriage.
The incident is believed to have taken place in the toilet between 5.45pm and 5.55pm, shortly before the pair alighted at Swindon.
The victim, aged in her 20s, is being supported by specially trained officers.
A man in his 20s has been arrested and released on bail while inquiries continue.
Detective Inspector Paul Stanley, of British Transport Police, said: "I'd like to appeal to anyone who may have been in the carriage at the time of the incident, or witnessed an interaction between a man and woman, to get in touch with us as soon as possible.
"I'd like to remind the public that incidents like this are thankfully very rare, and we will continue to conduct a number of additional patrols in the area to help reassure passengers."
Additional reporting by Press Association
