Network Rail is repairing damage to 500m metres of overhead wires.
GWR says work on the line between London Paddington and Ealing Broadway will limit the use of electric trains in this evening's rush hour, causing cancellations and alterations.
The Heathrow Express has also been suspended, with a replacement bus service brought in to transport travellers to the airport.
Passengers have been urged not to attempt to travel via London Paddington this morning, after a major power failure left some trains stranded on the tracks for hours.
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A spokeswoman said: "We are experiencing severe disruption on all services into and out of London Paddington and this is likely to affect services in other areas for the rest of the day".
A Hitachi spokesman explained the company was holding "a full and thorough investigation" to identify the cause of the damage.
Two of the four lines usually used were opened at around 12.40pm after engineers cleared debris and restored power so trapped trains could be moved but disruption was expected to continue throughout Wednesday.
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Class 802 trains are bi-mode, meaning they can operate on electric and diesel power.
Commuters have been "strongly" advised not to travel towards London today and GWR said a "reduced timetable" will be in place today which affects all mainline routes to and from London Paddington.
Delays on the Central, Circle, District, Hammersmith and City, Jubilee and Metropolitan London Underground lines compounded difficulties for passengers trying to get across the city after arriving at alternative rail hubs.
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