A 12 mile stretch of railway electrification was switched on between Heathrow junction and Maidenhead in a major milestone for Crossrail.
Electricity has been turned on in the first section, with the remainder to be turned on next year ready for summer 2017.
New Great Western Railway trains will run on the tracks as part of the Crossrail programme after a year of work.
More than three quarters of the wiring programme, which sees 150km of wires strung between the new equipment, has now been completed.
An 800-strong workforce has successfully installed more than 1,400 piled foundations and 834 overhead line structures.
The work is part of the Crossrail programme and Network Rail’s Railway Upgrade Plan to "provide a bigger, better, more reliable railway for passengers and businesses".
Matthew Steele, Network Rail project director, said: "This is a key milestone towards the introduction of brand new, cleaner, quieter electric trains on this very busy route into London.
"This electrification not only enables the introduction of the Elizabeth line but also supports the introduction of new GWR trains in 2017.
"I would like to thank the local residents and businesses for their patience as we undertake this sometimes noisy construction works."
Take a look at the Elizabeth line Crossrail
What is the electrification process?
Electrification to power the new trains includes installing overhead line equipment alongside and above the existing track.
It consists of overhead wires running between supporting steel posts to distribute electricity.
The steel posts are mounted on heavy duty foundations.
From summer 2017, GWR will be able to extend its electric train services to Maidenhead, which currently stop at Hayes & Harlington.
When the Elizabeth line opens fully in December 2019, passengers in the Thames Valley will be able to travel on new trains right through central London without having to change at Paddington.
Matthew White, Crossrail Surface Director, said: "This vital work is paving the way for quicker, greener, quieter and more reliable trains for people in the Thames Valley.
"Once the Elizabeth line opens fully, passengers will be able to travel right through the capital without having to change at Paddington, making it quicker and easier to get to a range of destinations across London and the South East."
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