STEEPLE CLAYDON, England — A chorus of bird song gives way to the roar of a chain saw and then the creaking and splintering of timber. A 50-foot tree sways, wobbles and finally crashes to the ground, while protesters shout and jeer.
The construction of the British government’s largest public works project — a high-speed rail line known as HS2 — has long been promoted as helping to save the environment. But it is under growing challenge from those who accuse it of doing the exact opposite.
They have waged a mostly fruitless fight against the project, a grand scheme to cut air and road travel by connecting the north of England to the more prosperous south with trains traveling at up to 225 miles per hour.
Now, with the pandemic prompting a surge in working from home and a slump in train travel, the opponents believe the argument is finally tilting their way, eroding the already shaky rationale for an effort that could cost more than $140 billion.
They include not just the hardened, young eco-warriors who camp among the trees near the ancient English wood of Steeple Claydon, hoping to stop construction, but also people like Clive Higgins, 71, the owner of a stretch of land in the path of the project, and a member of Prime Minister Boris Johnson’s Conservative Party. He said his generation was raised not to show emotion, but it was impossible when woodlands were torn apart.
“There are times when I crawl into a corner and cry,” Mr. Higgins said.
Tom Burke, a veteran environmental activist, formerly supported the rail line, which has been projected to appeal primarily to business travelers. Now he opposes it, citing the carbon footprint of the construction work itself, the threat to biodiversity and the pandemic-altered world.
“We are not going back to the same volume of travel on trains, people are not going to go back to work in the way they used to work,” said Mr. Burke, chairman of E3G, an environmental think tank.
The rail line would free up space on the current, creaking rail network, much of it dating from the 19th century, and it has the support of Mr. Johnson. He is a fan of prestige infrastructure projects and says he wants to spread prosperity to the north. Jobs are also at stake; at its peak the work promises to create 30,000.
But opponents dismiss it as a vanity scheme that, with the pandemic already requiring enormous spending, the country cannot afford. The money would be better spent, they say, on health or education, or on improving existing railroads linking northern cities to each other, rather than to London.
The first phase of the project would connect London to Birmingham, around 100 miles to the northwest. The next phase would push farther, with links to Manchester and Liverpool scheduled for completion between 2029 and 2033. A planned final phase would connect Birmingham to cities to the northeast, including Leeds, in Yorkshire.
The projected cost is immense — around £50 billion, almost $69 billion, for the first stages, and more than twice as much if it is extended to Leeds.
Travel time between London and Manchester is expected to be cut to 90 minutes, from 128.
On a sunny spring morning at Poor’s Piece, near the village of Steeple Claydon northwest of London, Mr. Higgins, the landowner and a former IT entrepreneur, said he had invited protesters onto his land after the project took part of it — so far without financial compensation — using rules that allow its temporary seizure. However, money is not what really motivates him, he said.
“We have planted and repaired wildflower meadows, we have recovered ponds, we have planted thousands of trees and planted miles of hedgerows and the reward I have got from a grateful society is just to come and kick it all to bits — all for no purpose,” he said.
Caroline Thomson-Smith, a hairdresser and former teacher, mounted a solo protest there last year, putting herself in the way of tree felling.
“It was heartbreaking because I knew that as soon as I was gone they would cut down the trees and I would come back the next day and they would be gone,” she said.
The project’s management says it is creating new wildlife habitats and planting seven million trees in the first phase of the work.
Opponents point to estimates that, including emissions from construction, the project would take 120 years to become carbon-neutral. Rail project officials say that figure is outdated and fails to account for new construction techniques, or fully reflect reduced road and plane journeys.
Britain’s green protest movement has stopped or slowed a host of road and other construction projects, but a victory over this one would be much harder. Trains are more popular and climate-friendly than cars or planes, construction of the first leg is already underway, and Parliament has authorized construction of one of its two northern sections.
But no decision has yet been made on whether, or when, to build the final, northeastern phase, so protesters hope that they can at least stop that final stretch.
Andrew Adonis, a member of the House of Lords, a former transport secretary and an architect of the plan, stands by it.
“If the pandemic had come five years ago there might have been a rationale for pausing it, but there is no argument when you have 250 construction sites between London and Birmingham and have spent £10 billion,” he said.
“Unless there is a dramatic change, there will be a need for significant new transport capacity,” he added, arguing that opposition comes from an alliance of nimbies and “fundamentalist greens who are against any development of any kind.”
The protesters complain of intimidation tactics from the project’s management and aggression from security staff. They also say the police use coronavirus regulations as a ruse for targeting campaigners.
Project officials reject those arguments, saying in a statement that “the activists, many of whom have already been arrested and are breaching their bail conditions, are well-organized and we have serious concerns that the level of criminal behavior could cause serious harm to our staff and the public.”
The protests are thought to have cost the project around £50 million already. Activists caught the authorities by surprise when they occupied tunnels dug near Euston Station in London, where the line starts and where Larch Maxey, a veteran of such protests, spent three weeks underground despite suffering from claustrophobia.
“I was living in an incredibly confined space, but it got better in the second and third weeks and it became an empowering experience,” he said in an interview. He described the project as “a 20th century scheme foisted on the 21st century,” adding, “The business model for HS2 was always shaky — it was based around the expected growth of business travel — and that has disappeared.”
At a protest camp at Jones Hill Wood, about 25 miles from Steeple Claydon, activists have built tree houses and other shelters on a landscape that inspired the writer Roald Dahl, and where tree felling was scheduled last year.
They say they have worked hard to monitor wildlife, including the location of badger dens and bat colonies, to hold officials to their promises to protect some species. But construction work is going on behind a green metal fence erected by security guards who take video footage on their phones of anyone who approaches.
Sitting around a campfire, Ross Monaghan, an activist who has spent a year here, much of it sleeping in a treehouse 80 feet above the ground, said it was “a victory that Jones Hill Wood is still standing, but we haven’t won that battle yet.”
To prevent more felling, he said, “people are going to have to step forward, put their bodies on the line, put their freedom on the line, and I think you will see that happen.”