Most train passengers in south Wales will need to use bus replacement services until New Year's Day due to engineering work on the main line.
Network Rail is renewing the track and installing overhead wiring between Cardiff and the Severn Tunnel.
Passengers going to or from London, north Wales, Manchester, the Midlands and England's south coast are all affected.
Buses will replace trains on affected sections of journeys.
Rail bosses have urged passengers between Christmas and new year to check their travel plans due to the amount of disruption to Welsh services.
London Paddington station is also shut on Friday so Great Western Railway services between south Wales and London will be by train between Bristol Parkway and Reading and by bus at either end of the journey.
GWR's London, Portsmouth, Taunton and Exeter routes will start and end in Bristol and buses will replace trains to Newport, Cardiff and Bridgend.
Cross Country services between Cardiff and Nottingham will start and terminate at Chepstow and buses will replace trains to Newport and Cardiff.
Buses will replace trains between Cwmbran and Cardiff on Transport for Wales (TfW) services between south Wales, north Wales and Manchester.
Engineering work also means there will be a bus replacement on TfW services between Ebbw Vale and Cardiff, and Cheltenham and Cardiff.