Last week, ATA reported that the work had sparked concerns about nesting birds.
He said he could not believe the severity of the work.
The RSPB told ATA they would also be reporting the incident to transport police.
The charity advises tree pruning and felling should be avoided at this time of year to avoid potentially harming birds.
However, a Network Rail spokesperson said “managing vegetation is one of our most important safety issues”, and the work complied with the Wildlife and Countryside Act 1981.
“We take our responsibility to protect the natural environment seriously, which is why an ecological survey was carried out prior to this work starting to identify any protected species or nesting birds in the area; checks are also made before work begins at the start of each day.
“If any species are identified, appropriate methods of working will be put into place.
“Last year, 1,000 trains collided with a fallen tree or large branch across the network, with many more instances of trees and branches blocking and closing lines, undermining safety and disrupting journeys for millions of people.
“With 20,000 miles of track and an estimated 2.5 million trees growing alongside the railway, managing vegetation is one of our most important safety issues and forms a key part of our Railway Upgrade Plan to improve reliability for passengers.”