Commuters are being warned to plan ahead and expect delays as the Severn Tunnel closes for six weeks for pre-electrification work.
The 130-year-old tunnel will close from Monday 12 September to prepare for a brand new fleet of electric trains.
Network Rail says the new trains will mean more seats and faster, more reliable journeys for passengers.
The work will affect people travelling across the South West of England, particularly routes in and out of Wales.
Passengers travelling to and from London Paddington will be diverted via Gloucester - an increased journey time of up to 35 minutes - which could see the regional station become much busier at peak times.
GWR high-speed train services will be diverted between Swindon and Newport, increasing journey times by around 30 minutes.
Meanwhile Network Rail has also issued a level crossing safety warning urging the public to be extra vigilant when using crossings, as there will be increased rail traffic on the line between Swindon and Gloucester.
Motorists will also need to allow extra time for their journeys as level crossing barriers will be down more frequently along the diversion line.