The incident happened on Platform 8 at 1pm on Saturday, February 16.
A man is reported to have hit a teenage girl in the face two to three times and then pushed her into a wall.
The girl was with three other children at the time of the assault.
The suspect is white with a heavy build, short dark hair, and had on a black jacket, grey jumper, lilac shirt and grey trousers.
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Officers would like to speak to anyone who witnessed the incident, especially the Good Samaritan who intervened.
You can contact the BTP by texting 61016 or calling 0800 40 50 40 quoting reference 295 of February 16, 2019
Alternatively you can call Crimestoppers anonymously on 0800 555 111.