Passengers travelling to and from London's Paddington station have been hit by delays and cancellations.
Heathrow Connect and Heathrow Express services, from Paddington to Heathrow airport, have been suspended.
First Great Western trains in and out of Paddington experienced cancellations and delays by up to 60 minutes.
The Tube is also experiencing delays. National Rail Enquiries is running updates on the situation (at time of writing normal service is expected to resume at 15.00) on their website.