This a question sometimes posed by those planning new high speed rail networks.

An airport is clearly a major traffic generator - and, probably, the larger the airport the larger its traffic potential.

At the same time, so is a city centre, a downtown, a Central Business District.

While it is true that these two points - airport and city centre - are not everyone's destination, it is likely that a city centre is (almost by definition) more central to a city region than an airport. Equally, it is often important for regeneration reasons to include a city centre stop.

What's the impact on journey time (and therefore attractiveness) of doing both - of stopping at both airport and city centre?

One fundamental assumption is that this can be done without significantly lengthening the route.

I did three calculations, based on an average speed of 250 km/h and a distance of 1010 km. I assumed that the first and last five kilometres would be limited to 50 km/h.

From this, it can be calculated that a non-stop journey would take four hours and 12 minutes - 4 hours for the 1000 km at 250 km/h and 12 minutes for the 10 km at 50 km/h.

Supposing that there is an airport 15km from the city. Those 15 km are limited to 100 km/h (5 km/h faster than Heathrow Express's average start-stop time). The station stop itself takes 2 minutes.

Under those assumptions, the 12 minutes would still be required for the terminal stations at each end. The line speed of 250 km/h could only be used for 985 km: the remaining 15 km would be done at 100 km/h. This gives a total journey time of 4 hours 19 minutes - 7 minutes longer than the non-stop journey.

The third calculation assumed that the airport was 30km from the city, and that those 30km are limited to 100 km/h.

Under those assumptions, the 12 minutes would still be required for the terminal stations at each end. The line speed of 250 km/h could only be used for 970 km: the remaining 30 km would be done at 100 km/h. This gives a total journey time of 4 hours 25 minutes - 13 minutes longer than the non-stop journey.

Finally, I tested the impact of reducing the overall distance - to 510 km rather than 1010.

This reduced the total journey time in all three scenarios by two hours, so the percentage incremental time caused by the airport stop is larger, but the absolute amount is not.

With the longer journey, the percentage increases in journey time are 3% and 5%: with the shorter one, they are under 6% and 10%.

So there is certainly a time penalty. Are its effects offset by the increased revenue from serving the airport directly? Is this greater than the reduced revenue caused by attracting fewer centre-centre passengers with the longer journey time?

Suppose a 10% increase in travel time leads to a 10% decrease in passenger numbers? Would that be offset by the revenue from airport-specific passengers? What percentage increase in passenger numbers would you get from a stop at the airport?

Only specific studies in specific localities will show this with any degree of accuracy. But it shows the kind of size of the effect we may be considering here.