From Monday 18th February 2019, Whiteball and Marley Tunnels in Devon will start undergoing important maintenance upgrades. This will mean fewer trains to London on the West of England line.
Journey times will also be extended by up to an hour with diversions in place between Castle Cary and Exeter St Davids. Rail replacement transport will also run between Taunton and Exeter St Davids.
Full details can be found here https://www.gwr.com/travel-updates/planned-engineering/swupgrade including a journey check feature for checking specific journeys.
The work will be ongoing for three weeks and the revised timetable will therefore be in place until Friday 8th March 2019.