From Monday 12 September to Friday 21 October 2016, the Severn Tunnel will be closed to enable electrification through to Cardiff and Swansea.
The three main stations affected are Bristol Parkway, Severn Tunnel Junction and Newport.
Newport to London services will be diverted and replacement buses will run between Newport and Bristol Parkway.
Alternative timetables will operate, with journeys from Newport to London taking about 35 minutes longer and Newport to Bristol 45 minutes longer.
Throughout the period the rail replacement coaches will be running between Bristol Parkway and Newport as well as Bristol Parkway and Severn Tunnel Junction, with multiple coaches and standby vehicles on all peak services.
Arrival in Parkway and Newport is set to meet the majority of connections where a full train service will run.
There will be extra customer service staff at all locations with information on connections.