Police are appealing for women who may have been victims of sexual assault on trains between Lichfield and Birmingham to come forward.
Three women have reported being touched inappropriately by a man on July 1, August 16 and September 28.
Officers say they believe the incidents are linked and have urged any other potential victims to come forward.
Sergeant Wayne Riley from British Transport Police said: “We take all reports of unwanted sexual behaviour extremely seriously and our investigation into what happened is progressing.
“A 34-year-old man was arrested on September 30 on suspicion of sexual assault and questioned over these allegations. He has since been released on police bail while enquiries continue.
“It is possible there are other people who have been victims of sexual offences who have not yet spoken to us. I would encourage anyone who has been subjected to or witnessed similar behaviour, particularly while travelling on the Lichfield to Birmingham line in recent months, to contact us.”
Anyone with information is asked to call officers on 0800 405040 or text on 61016 reference 205 21/10/16.