The incident took place at about 11.42am on Sunday, 21 December on the 11.22 Metro service from Sunderland.
PC Nick Cannon said: â€œThe service was extremely busy carrying a large number of football fans to the Sunderland V Newcastle derby game.
â€œAs the train pulled into Fellgate Metro station one of the train windows was smashed.
Passengers were moved out of the carriage in to the rear of the train, which eventually then had to be taken out of service to be fixed.
PC Cannon added, â€œThis was a mindless act of vandalism. If you were in the carriage and saw the person responsible or have any information that may assist our enquiries into this crime please get in touch.â€
If you can assist the investigation in anyway please call British Transport Police on 0800 40 50 40 or 61016 or Crimestoppers, anonymously, on 0800 555 111. In all calls please quote log PSUB/B5 of 5/1/2015.