The incident happened on Thursday, 23 October, at about 10.40am as the Virgin Trains service travelled between the two stations.
PC Michelle Johnson said: â€œA member of staff briefly left a mobile phone on a seat in a carriage of the train.
â€œWhen they returned to the seat the phone had been taken.â€
Officers have viewed CCTV from the train and have isolated the image of a woman they would like to speak to.
PC Johnson added: â€œI am certain the woman in the CCTV image has information about the theft and will be able to help our enquiries â€“ but who is she?â€
If you can help please call British Transport Police on 0800 40 50 40 or call Crimestoppers, anonymously, on 0800 555 111. In all calls please quote log PSUB/B4 of 4/12/14.