A train has derailed west of Melbourne, causing major delays for peak-hour traffic.

The Department of Transport confirmed a freight train had derailed in North Geelong at about 1.30am on Thursday.

Roads have been closed in all directions between Separation St, Dunn St and Douro St.

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Boom gates along the affected roads are down, with commuters urged to find alternate routes.

It is not yet known what caused the train to derail or the extent of the damage.

There are currently no injuries reported.