Eight weeks of disruption begins this slot weekend for rail passengers as Network Rail replaces tracks and signals.
Some services to and from Bristol Temple Meads will be replaced by bus journeys, starting on 10 July, and some timetables will change.
In late August the station will close completely for five days.
The Bristol Rail Regeneration programme also involves renovation of the Grade I station including roof repairs and more shops will be added to the station.
Network Rail said the new tracks and signalling would increase capacity on lines in and around Bristol, allowing more trains to be added to the network.
Between 10 July and 6 August there will be no direct trains between Bristol Temple Meads and Cardiff or Gloucester, and buses will replace trains between Temple Meads and Bristol Parkway and Temple Meads and Severn Beach.
On the weekend of 6 and 7 August there will be major changes including to services from London Paddington to Bristol and Portsmouth to Cardiff Central.
Throughout most of August there will be disruption to services between Bristol and Bath Spa.
No trains will run to or from Temple Meads between 31 August and 3 September.