'Essential' work brings five weeks of train disruption
Rail passengers are being urged to check train timetables as work could affect travel across the south Wales mainline for five weeks.
The "essential" upgrade affects the line between the Severn Tunnel and Cardiff from Monday until 25 May.
Replacement buses will be in operation on some routes with timetable revisions on less frequent services.
Network Rail said it was working with rail companies such as Arriva Trains Wales (ATW) to "keep people moving".
Among the changes, ATW's Manchester and Holyhead services will see replacement bus services between Cardiff and Cwmbran as well as Newport and Cwmbran.
There will also be replacement buses only for travellers using the Ebbw Vale line.
Network Rail project director Dan Tipper said: "This year we reach a critical stage of our work to modernise the south Wales mainline.
"We are now focusing on the installation of the overhead line equipment between London and Cardiff that will power the new bi-mode trains."
The transport can run on electrified sections of track and transfer to non-electrified sections.