20,000 staff are working to upgrade the rail network across the UK and here in the south west.
There have been no services into Paddington for the last four days and the line remains closed today with services diverted to Waterloo and Marylebone.
From tomorrow (Dec 29) half the lines will open but there will be disruption until Sunday.
Journeys will take up to 90 mins longer from the region to London. There is also work at Swindon and Bath. The work continues until Sunday.
Toufic Machnouk, Network Rail Project Manager, says over the holidays is the best time to carry out work because there are less passengers.