Engineering work will affect train travel between and Pwllheli and Machynlleth this December.
Buses will replace trains between Machynlleth and Pwllheli stations from 4.45pm on 8 December to 7.30am on 16 December.
Passengers are urged to check before they travel between the two places when Network Rail carry out essential engineering work on the Cambrian Line.
Network Rail engineers will replace more than 1km of track between Dyffryn Ardudwy and Harlech stations and install 1,800 new sleepers and 3,600 tonnes of ballast (track stone) using an auto hopper train to be refilled in Barmouth.
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.
On 8 December, the 16:52 and 18:55 Machynlleth-Pwllheli services will be replaced by buses.
From 9 December to Sunday 15 December, buses will replace trains all day.
On 16 December, the 06:29 and 07:29 Pwllheli-Machynlleth services will be replaced by buses.
These replacement buses will run earlier than usual to connect with train services at Machynlleth.
Details are available in journey planners on the Transport for Wales website and passengers are advised to check it before travelling.