A third runway at Heathrow is being presented as an engine for growth. But before the first aircraft takes off from the new strip of concrete, another question is becoming increasingly difficult to ignore:

How will all those extra people get to the airport?

Heathrow’s proposed £33bn expansion could eventually add between 100,000 and 200,000 passenger journeys to and from the airport every day. Hundreds of additional flights would fill the skies, while thousands more workers and passengers would have to cross west London on the ground.

The airport already knows what it means to make room for another runway. Its plans involve moving a section of the M25 and constructing a new 2.2-mile runway.

The harder task may be moving people.

For years, the Heathrow Express has been marketed as the premium answer: Paddington to the airport in as little as 15 minutes, with luggage racks, toilets and the convenience of a dedicated service.

But speed comes at a price.

A standard one-way ticket can cost £26, while the service carried only around 4.5 million passengers last year. Compared with Heathrow’s overall passenger numbers, that is a relatively small share — and the figure includes airport employees travelling on heavily discounted fares.

Meanwhile, another railway has quietly changed the economics of getting to Heathrow.

The Elizabeth line now reaches deep into London, connecting the airport with areas such as the City and Canary Wharf without requiring passengers to make the traditional journey to Paddington first.

Its central-London fare is £15.50, while the Piccadilly line offers an even cheaper route at about £5.90.

That has left the Heathrow Express occupying something of an awkward middle ground: faster for some passengers, but significantly more expensive and increasingly difficult to justify when railway capacity is desperately needed elsewhere.

Transport for London believes those precious train paths could be put to better use.

Alex Williams, TfL’s chief customer and strategy officer, told MPs that Heathrow Express services were occupying “valuable paths” that were not being used particularly heavily. His argument is straightforward: those paths could potentially be repurposed into services that are cheaper, more accessible and capable of carrying substantially more passengers.

That matters because the Elizabeth line itself still has weaknesses.

Only two trains an hour currently serve Terminal 5, despite the terminal handling roughly 40% of Heathrow’s passengers. Frequency is expected to increase, but expanding Heathrow could create demand far beyond what the existing network can comfortably absorb.

And the problem is bigger than one train service.

Britain is trying to expand an airport whose surface transport system is already under pressure.

Two other rail proposals — Heathrow Southern Railway and a Western Rail link — could connect the airport more effectively with the wider national network. There has also been discussion of extending the Elizabeth line towards Staines.

Together, such projects could give passengers from London’s suburbs and the surrounding commuter belt more reasons to leave their cars at home.

That is becoming increasingly important because Heathrow’s expansion plans also include more parking.

The airport intends to increase parking capacity from around 21,000 spaces to 36,000.

That creates a strange contradiction.

If Heathrow makes it easier to drive almost directly to the terminal while rail capacity remains constrained, what incentive is there for passengers to choose public transport?

This is where the Heathrow Express debate becomes much bigger than the fate of one premium train.

It is really a question about what kind of airport Heathrow intends to become.

The airport says Heathrow Express generates revenue without taxpayer funding and helps keep airport charges down. It also argues that passengers value having a choice.

But critics see the railway capacity occupied by the service as an opportunity cost. Every train path devoted to a lightly used premium service is a path that cannot easily be used for a higher-capacity alternative.

The timing makes the issue even more significant.

The Heathrow Express’s current access agreement with the Department for Transport expires in June 2028. At roughly the same time, Britain’s railway system is undergoing major structural change, with Great Western Railway due to move into public ownership and eventually become part of Great British Railways.

In other words, the decision about Heathrow Express is arriving just as the wider railway system is being redesigned.

And Heathrow itself owns the stations and roughly five miles of railway between the airport and the national network, infrastructure that dates back to the original investment behind the Express in the 1990s.

The numbers are therefore only part of the story.

The larger question is whether Heathrow’s expansion can be separated from the transport system around it.

London Mayor Sadiq Khan has warned that taxpayers could ultimately be asked to fund billions of pounds of transport improvements that remain undefined or unfunded within the expansion proposals. TfL has estimated the potential requirement at up to £9bn, although Heathrow disputes that figure.

Meanwhile, Heathrow is proposing to spend billions expanding the airport — including increasing parking capacity — without committing a comparable share of its £33bn expansion budget to improving rail services.

That has intensified concerns about what happens when the extra passengers actually arrive.

The government’s consultation does include public transport requirements, and officials say future planning permission would have to meet transport targets.

But critics argue that targets which only become fully relevant once the third runway is operating could arrive too late.

Anthony Smith, who chairs the Heathrow Area Transport Forum, put the concern bluntly when speaking to MPs: trying to persuade people to abandon their cars after Heathrow has reached dramatically higher passenger numbers would make the task considerably harder.

That is perhaps the central lesson hidden inside the Heathrow Express argument.

The choice may not ultimately be between keeping one train and cancelling another.

It may be between designing Heathrow’s transport system before expansion locks in new patterns of travel — or trying to repair the consequences afterwards.

A third runway would change Heathrow’s capacity in the sky.

The more consequential question for London may be whether the railway beneath it changes quickly enough too.