Trains will stop running between Bristol's two main stations from today until mid-November.
The route between Bristol Temple Meads and Bristol Parkway and along the Severn Beach line is being shut for improvement works.
Network Rail is laying new track for two additional railway lines to ease a historic bottleneck.
A spokeswoman said the work would transform travel for passengers across the city.
Stations remain open
Buses will replace trains between the stations affected, and Bristol Temple Meads and Bristol Parkway stations will both remain open.
Customers travelling towards Bath Spa from south Wales, Bristol Parkway, Cheltenham Spa or Gloucester can travel via Swindon instead of Bristol Temple Meads.
Network Rail project manager Kate Trevorrow said of the improvements: "It will enable GWR to run more frequent and quicker services between Bristol and London Paddington, providing more seats and better journeys.
"It will also ease congestion on the network between Bristol Temple Meads and Bristol Parkway."
The additional lines will provide two tracks in each direction along the route from early 2019 and are aimed at improving the punctuality and frequency of services.
The contractors are laying the track between Lawrence Hill and Filton Abbey Wood stations, which means stations between Parkway and Temple Meads - as well as services towards Severn Beach via Clifton Down and Avonmouth - are out of action until 18 November.