Buses will replace trains between Machynlleth and Pwllheli stations from 4.45pm on Sunday 8 December to 7.30am on Monday 16 December.
Network Rail engineers will be replacing more than 1km of track between Dyffryn Ardudwy and Harlech stations.
They will install 1,800 new sleepers and 3,600 tonnes of ballast (track stone) using an auto hopper train which will be refilled from a virtual quarry at Marine Parade, Barmouth.
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Nick Millington, Network Rail Wales and Borders route director, said: "Our teams will be working around the clock to carry out this important track renewal on the Cambrian Line. The age of the track and its exposed location on the Cambrian Coast means we must replace it this winter.
"We recognise there is never a good time to close the railway for more than a week. But this allows us to minimise wider disruption and do the work in a more cost-effective way than if we had to return over several weekend shifts.
"I would like to thank passengers for their patience and understanding and urge them to check their journeys before travelling."
Transport for Wales (TfW) will run a mixture of fast and stopping buses.
