An assault on a man at Harpenden station was captured on video and is circulating on social media.
Police were called to a fight on the platform at 3.26am on January 25.
A 21-year-old man was taken to hospital where he received treatment for a broken nose.
Three men, aged 18, 21 and 22, were arrested in connection with the attack.
They have been released under investigation while police enquiries continue.
Anyone who witnessed what happened and hasn't already spoken to police is asked to contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 525 of 26/01/20.
Alternatively you can call Crimestoppers anonymously on 0800 555 111.
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