Thames Valley Transport Police are asking for passenger who reported the fight to the train guard before getting off the train at Tring station to come forward.
They believe this person may have information to assist their investigation.
It's in connection with their investigation to a fight on board a service travelling between London Euston and Leighton Buzzard railway stations.
At around 10pm on Sunday 16 May a fight broke out between three men on the rear carriage of the train.
The train was held at Leighton Buzzard station while an off-duty officer dealt with the incident. It was then moved to Bletchley railway station where officers arrested two males in connection to the incident.
Any witnesses or anyone with information can contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 540 of 16/05/21.
Alternatively, you can call Crimestoppers on 0800 555 111.
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