Heathrow Third Runway Is a Rail Slot Problem
Heathrow’s third runway debate is usually framed as a question of concrete, sky and noise. The binding constraint sits elsewhere: on the railway tracks that feed Britain’s busiest airport.
The scarce capacity at Heathrow is not concrete and not runway. It is track.
Heathrow’s two runways sit ringed by the M25, the busiest motorway in the country, and by reservoirs and orbital roads. Adding a third runway means adding a 2.2-mile (3.5 km) strip and shifting part of the M25 to make room.
That is the plan Heathrow has put forward, at a stated cost of £33bn. If it is built, hundreds more planes would cross London’s sky each day. On the ground, between 100,000 and 200,000 additional passengers would eventually need to reach or leave the airport daily, and thousands more staff would need to do the same. The sky is the easy part of that arithmetic. The roads are not.
Moving a motorway is the eye-catching problem. Getting people out of their cars is the thornier one. Ride-sharing, coaches and buses can absorb some of the load, but rail is the only alternative with the volume to matter. Rail, unfortunately, is where the capacity is thinnest.
The constraint is a slot-allocation problem rather than an engineering one. The question is not whether the Heathrow Express is a pleasant train, but what else that path could carry. Transport for London has made its view plain: the service’s future is “worth reviewing” in the context of the optimal mix of services on the Great Western mainline. Alex Williams, TfL’s chief customer and strategy officer, told MPs the Heathrow Express was “taking up valuable paths that aren’t actually well used”. His proposed fix is repurposing rather than abolition for its own sake: more affordable, more attractive, more public transport users.
The numbers behind that judgement are not flattering to the premium option. A one-way Heathrow Express ticket costs £26 for a 15- to 21-minute hop. It is the fastest way to Paddington, and it comes with a toilet, a luggage rack and TV news. But last year it carried only 4.5 million passengers. That is less than 6 % of Heathrow’s air passengers at best, and the figure is flattered by airport staff on heavily discounted tickets. The Elizabeth line charges £15.50 to any of its central London stations and runs deeper into the City and Canary Wharf. The Piccadilly line charges £5.90 into central London. Advance Express tickets booked a month ahead can cost £10, which narrows the gap in the other direction. Heathrow says the service generated £48m of revenue in the first six months of 2026.
Capacity, though, is the crux. Only two Elizabeth line trains an hour currently serve Terminal 5, which handles about 40 % of the airport’s passengers. With poor timing, the journey from the City or Canary Wharf can stretch beyond an hour. The frequency is due to double next year, which eases pressure without dissolving it. More space into Heathrow could still be released.
Surface access to Britain’s biggest airport changed step with the Elizabeth line. Before it, a little-advertised service called Heathrow Connect stopped at places like Hayes & Harlington on a slower run to Paddington. It served plenty of airport workers, if fewer international travellers, and its fares ran at roughly a third of the Express price. That gap has since narrowed considerably.
Following the money clarifies the decision-making. Heathrow owns the Express stations and five miles of railway from the airport to the mainline, part of a £1bn investment in the 1990s. The operator is FirstGroup-owned Great Western Railway, due to be nationalised this December and folded into Great British Railways next year. The access rights that let the service run on the national network come from the Department for Transport and expire in June 2028. The DfT says no decision has been made on future operation. MPs must approve the national policy statement before any runway proceeds, and that approval no longer looks automatic. Andy Burnham’s long-standing scepticism about a third runway has not softened. The loss of Rachel Reeves from Number 11 removed a champion of expansion from the Treasury. The new chancellor has said the economic case stacks up, but later rather than sooner. Downing Street support looks ambivalent at best.
Meanwhile the ground-side plan leans heavily on cars. Heathrow is funding new car parks, with capacity planned to rise from 21,000 spaces to 36,000. It has not pledged any of its £33bn on improving the trains, Williams told the select committee. His question was blunt: if parking is made easy right beside the terminal, what encourages anyone to board a train? Sadiq Khan, the mayor of London, warned that taxpayers could be left paying for public transport improvements “left undefined and unfunded” in the proposals. TfL puts that bill at up to £9bn. Heathrow says it does not recognise the figure.
The current policy statement sets public transport targets rather than a legal mandate. At their most stringent, they envisage 55 % of airport passengers using public transport, and that applies only once a third runway is fully operational. Williams called the absence of a clear mode-shift requirement “a step backwards”. In 2017, he noted, there was a commitment or at least an aspiration from Heathrow for no extra traffic on the roads. That is gone. Gatwick’s recently approved second runway, by contrast, carries a mode-shift condition in its planning permission.
Two further rail schemes have been floated: Heathrow Southern Railway and a Western Rail link into the national network. The Elizabeth line itself could be extended to Staines. Each would pull more of the suburban commuter belt onto trains instead of into cars.
Anthony Smith, a former passenger watchdog who now chairs the Heathrow Area Transport Forum, sees the timetable as curious. His warning to MPs: shifting people out of cars in 20 years’ time, with 130 million passengers, gets “harder, harder, harder”. For now, these are only targets.
That is why June 2028 matters more than any glossy rendering of a new runway. That month the Heathrow Express’s access rights to the national railway run out, and with them the easy default. A consultation is open now, MPs must vote on the policy statement, the operator changes hands in December, and the Terminal 5 frequency doubles next year. Something has to be settled inside that window. The paths will not wait.
Mentioned organisations (context, not sources)
- Microsoft — Organisation (homepage)
- Apple — Organisation (homepage)
- Tesla — Organisation (homepage)
- Nvidia — Organisation (homepage)
- Heathrow — Organisation (homepage)
- Transport for London — Organisation (homepage)
- Heathrow Express — Organisation (homepage)
- Great Western Railway — Organisation (homepage)
- Department for Transport — Organisation