People living near the railway line between Monmouthshire and Cardiff can expect "several months" of noise and disturbance, Network Rail has said.
Work is under way to install foundations for steel masts that will carry electrified wires along the south Wales mainline.
The company said piling was a "noisy activity" that involved driving steel piles into the ground.
It aims to electrify the line between London and Cardiff by December 2018.
This would shave about 14 minutes off a typical journey.
Network Rail's Andy Thomas thanked residents for their patience and said disruption would be kept to a minimum.
New bi-mode trains are set to run on electric and diesel, providing extra seats and a faster service.
Earlier this month the UK's transport secretary Chris Grayling said plans to electrify the line between Cardiff and Swansea had been scrapped.