Rail passengers returning to work after the Christmas holidays are being warned to expect "significant disruption" due to strike action throughout the week.
Members of the RMT union and Aslef drivers will be walking out over pay, jobs and conditions.
Services on the Welsh Marches line won't run today, tomorrow and Friday and Saturday as around 40,000 Rail, Maritime and Transport members on Network Rail take industrial action.
Transport for Wales said none of its drivers are striking but says it can't run trains because of the action on Network Rail.