Problems near Hayes and Harlington are causing delays of up to 45-minutes between the two stations.
The delays are affecting travellers in Maidenhead and Slough and on the Heathrow Express.
There were delays of up to two hours earlier.
First Great Western passengers are allowed to use Chilten Railways, Cross Country, South West Trains, Virgin Trains and London Underground services.
For more information, go to www.nationalrail.co.uk
Updates to follow