‘Personally, I think that’s quite cool!” David Brown, 60, is beaming like a young boy, having just recognised the bus controller at the terminus outside Victoria station as a colleague who joined London Transport at the same time as him, almost 40 years ago. “People stick in transport a long time. That’s what I love about it. They’re doing a frontline job, I’m just doing mine, there’s no difference really.”

Except Brown is trying to steer not just buses but a multinational transport group as chief executive of Go-Ahead – in particular, to wrestle its emissions down to net zero, as the sector faces up to being the biggest contributor to greenhouse gases. This year he will leave the group – whose operations include Thameslink, Southern and Southeastern trains and buses in London and nationwide – after a decade at the helm.

While Covid threatens to unravel a lot of the work done to build up rail and bus services during Brown’s career, he is clear that climate change is the bigger long-term issue. Transport has far surpassed energy generation as the biggest CO₂ culprit – making up a quarter of UK emissions – and last week Go-Ahead made a pledge that its 5,000 UK buses and trains would be entirely zero-emission by 2035, cutting its CO₂ by 75%. It aims to hit net zero by 2045, before the national target, by offsetting the remainder.

Although Go-Ahead’s decarbonisation strategy – edged off stage by the government’s, which was published the same day – sets out many ambitions, it admits that many are not in its own hands. So what exactly is the point?

“It’s galvanising 30,000 people to get behind a climate strategy,” says Brown. “It’s a sense of purpose. What we deliver is helping solve climate change problems – if you get people on to public transport you’re taking them out of their cars.” About 55% of transport emissions are private cars, he says; just 3% come from buses, and 1% from trains.

The pledges assume continued government spending on hydrogen and electric vehicles, and subsidy for green operations. Brown lobbied for a change announced in the government’s decarbonisation plan, improving bus operators’ grants for running electric vehicles to 22p per kilometre. “It transforms the economics for investing in new buses.”

He thinks there are opportunities for more hydrogen buses, but is cautious: “The capital cost is huge and it’s unknown what the ongoing operating costs and lifetime costs will be.”

Go-Ahead’s north London depot at Northumberland Park will be what Brown bills as “the first bus-to-grid virtual power station”, where electric buses charge slowly overnight, and put energy back into the network from their batteries when supplies are needed, as wind and solar supplies – and prices – fluctuate.

In all this, as the small print of the strategy makes clear, there is a commercial imperative: “If Go-Ahead does not take action on this issue, our competitors will – and those with more climate-friendly reputations could ultimately take market share from us. This would weaken our business.”

Brown happily concurs. “There’s an altruistic view, and a commercial reason for doing it, in terms of positioning. And a people reason: younger people especially are attracted to work for companies who have purpose and are doing the right thing environmentally.”

Right now, though, public transport faces a more immediate crisis, with passenger numbers still only about half of pre-pandemic levels. And there is a renewed focus on the risks with Covid cases soaring, particularly as mask-wearing becomes optional on trains in England.

Brown frowns. “Whenever anyone talks about a tight, packed environment, they talk about public transport – and I want to scream and say hold on, the average journey time on a bus is 18 minutes max, the doors are opening all the time, fresh air is coming in and out, the windows are open on the top deck. You really aren’t exposed as you would be in a packed pub sitting there for two hours, there’s no comparison.”

He doesn’t mention names, but the prime minister, Brown’s former boss when mayor of London, suggested even as he was removing the legal requirement to wear masks that people “might choose to do so in enclosed spaces, such as public transport”.

Brown argues: “There seems to be a little bit of demonising it and that shouldn’t be the case. There is no evidence that anyone can catch Covid on a train or a bus, none whatsoever.”

That conviction comes despite the Covid deaths of a significant number of bus drivers. Brown says Go-Ahead believes none contracted Covid at the depot or while working.

Another factor may be at play, he suggests, comparing the clamour to travel abroad on planes, which are more enclosed than buses or trains: “People are choosing to do that because the prize at the end is going on a holiday. They might not be choosing public transport because the prize at the end is going to work.”

He sees a similar phenomenon with rail: “We have much busier trains at the weekend now, people are going to the coast – they love it, they don’t worry about what’s happening in the trains in those circumstances.”

On the mask issue, he says, he wants transport “to be treated the same as other parts of the economy”. If he could choose, “I’d want to say, everyone should be doing it everywhere, in any environment, I want that consistency”. Come Monday, he will still wear a mask. “It’s not protecting you, it’s protecting other people … it’s just a polite thing to do.”

Covid, he says, has only accelerated underlying changes towards working from home and ordering goods. “I don’t think we’ll go back to packed trains, because social trends are changing. Commuter journeys are going to become more discretionary.”

But net-zero targets depend on people returning to public transport, rather than the car, he says. “We have to find ways of getting people back on the railways, and we have to tackle the costs, because the cost base is not sustainable now.”

However, he points out that public transport is often seen abroad as part of the “fabric of society”, and subsidised accordingly. “You need to cut your cloth, attract customers – and you may need government money, because of the social benefits.”

Nowhere is this more apparent to him than in the capital. “We used to bring 150,000 people into London Bridge every morning. They’re not coming at the moment. That affects everyone. The big fear I have for places like London is how do you keep that vibrancy of the city centre, if you don’t have all those people coming in? You need all that activity and buzz – otherwise, you’re just in the suburbs.”

It seems inconceivable to remember, he says, that in the job-scarce 1980s, when he started as a graduate trainee, the discussion at London Transport was about cutting back the Bakerloo and the Northern lines because the population of the capital was in decline.

But without public transport, “it wouldn’t move, it wouldn’t function”. The challenge now for operators, he says, is “making sure that when people do come back, that we’re ready and we’re there for them. If they don’t find that the 7.25am is still operating or we don’t have the same frequency of service, then we’ve got a problem.”