Train passengers will face slower journeys between Bristol and London Paddington over the next two weekends.
Network Rail is preparing to undertake work between Bristol Parkway and Didcot Parkway on February 29, March 1 and March 8.
Journeys between Bristol Temple Meads and London Paddington will take up to 30 minutes longer as trains run on alternative timetables.
Buses will replace trains on March 8 between Gloucester, Bristol Parkway and Bristol Temple Meads.
Great Western Railway project manager Steve Bale said: "This vital track upgrade work will ensure that we can operate more reliably and continue to deliver the performance we know our customers expect."
While the work takes place, a shuttle train service will run between Bristol Temple Meads and Swindon (calling at Bath Spa and Chippenham), with a shuttle service also operating between Cheltenham Spa and Swindon.
Connecting coaches will operate between Reading and Swindon, as well as between Didcot Parkway and Swindon.
