First Great Western is revising its timetables between the West Country and Wales due to £4m of maintenance work.
The train company says it wants to remind customers of the changes
The repairs are to tackle flooding in the Chipping Sodbury tunnel between Swindon and Bristol Parkway.
The line will be closed from 1000 on Monday to 1400 BST on Friday during the first three weeks of August.
Travellers between Wales, Parkway and Swindon will be most affected, but commuters using Temple Meads are advised to check the revised timetable.
Andrew Hughes, from the train company, said: "We have publicised this through station posters, our website, e-mail bulletins, on-train letters, stakeholder communications and the media.
"We know information is key to our customers, so we're reminding them now about this extensive project."
Services from South Wales to Paddington, calling at Bristol Parkway and Swindon will operate on their usual route on each of the three Monday mornings, as will services returning on the Friday evenings.
Other service changes include:
Bristol Parkway: From Tuesdays to Fridays, peak morning services to Paddington will call at Bristol Parkway, with the last departure at 0759 BST.
Some peak evening services from Mondays to Thursdays will also call at Bristol Parkway starting with the 1630 BST departure from London Paddington.
All of these peak services will be subject to extended journey times.
Off peak services will call at Patchway, instead of Bristol Parkway.
Bristol Temple Meads, Bath Spa, Chippenham, Swindon, Didcot Parkway and Reading: Services will continue at the same frequency with some new times.