All rail services between London and Reading were halted due to a trespasser on the tracks.
Trains to and from Paddington were stopped due to the incident, which came after the country's train network was struck by cancellations and delays over the weekend due to Storm Ciara.
The Great Western Railway Twitter account posted just before 9.30am to announce the disruption, later posting that services were up and running but with delays and cancellations.
It said: "There is a trespasser on the railway and all lines between #Reading and #LondonPaddington are currently closed as a result."
They later added: "#LondonPaddington - All lines have been reopened following a trespass incident between London Paddington and Reading / Heathrow Airport.
"Trains may be cancelled, delayed by up to 45 minutes or revised as services return to normal."
The Paddington to Reading route is one of the busiest commuter lines heading into the capital.
There are hopes that once Crossrail is introduced some of the pressure will be alleviated.
Heathrow Airport is also on the Paddington line, and the London hub serves as the terminus for the express train service.
