Network Rail are doing some emergency engineering work tomorrow (Sunday 25th June) in Patchway Tunnel.
Services to and from South Wales will operate via Stroud, Gloucester and Chepstow. Some services from Wales will leave earlier to maintain on time arrivals in England, while those heading west will arrive in Wales later than published.
Some Golden Valley services will be retimed to make room for the extra trains using that route.
Some
HSTs▸ to and from Bristol Temple Meads will be extended to and from Bristol Parkway.
Full details here -
https://www.gwr.com/travel-updates/planned-engineering/25-june-2017