From journeycheck:
Cancellations to services between London Paddington and Slough
Due to a fault with the signalling system between London Paddington and Slough some lines are blocked.
Train services running to and from these stations may be cancelled, delayed by up to 60 minutes or revised. Disruption is expected until 13:00 19/08.
Customer Advice
South West Trains, London Underground, Cross Country and Chiltern Railways are conveying passengers via any reasonable route until further notice.
Additional Information
A freight train trying to access Southall Sidings has come to a stand accoss a set of points blocking lines between London Paddington to Slough.
DB» Cargo accepted a freight train into their yard at Southall onto a line which was already occupied by another freight train, end result being it didn't fit in the yard and therefore it blocked all 4 main running lines.