The improvements include an additional track installed for one mile either side of Darwen station, providing capacity for a more efficient service.
Due to the upgrade works, no train services will operate between Blackburn and Bolton from 17 July until 24 August.
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Darwen Borough Council along with the Lancashire Local Enterprise Partnership and Blackburn will fund the improvements.
Terry Strickland, Network Rail's Area Director said: "This essential upgrade will provide a better, more reliable railway for passengers travelling between Manchester and Clitheroe for many decades to come. We have worked closely with Northern Rail so passengers are still able to get to their destinations during this essential work which can only be done when trains are not running."
Passengers are being urged to check the latest information before travelling in order to avoid delays.
(LM)
