I enjoyed this article in the New York Times of 15 March (see the web-site of the New York Times).

The only point I take issue with is the observation that the main reason why there are still flights between Madrid and Barcelona is the entry of low cost carriers like Ryan Air (sic).

I think it is more likely that these flights are carrying interlining passengers - those flying from Barcelona to Madrid to change to planes to other destinations.

If - as has been talked about for some years - there was a high speed train service between Barcelona and Madrid Airport, I think many of those flights would stop. Passengers would interline between high-speed train and long-haul plane, rather than short-haul plane and long-haul plane. This would follow the example of Amsterdam - Antwerpen, Brussels - Paris and Frankfurt - Koln.

The station at the airport's new Terminal 4 has provision for high speed trains - there are 2 platforms beside the Metro platforms with no tracks just waiting for the day!

It is received wisdom that passengers don't change airports when changing planes. There is sometimes a plea for a rail connection between Heathrow and Gatwick, for example, which is always rejected on the grounds that very few people fly into one airport and change to a plane flying out of the other. I have seen figures for this particular transfer and they are pretty low.

However, travelling back from Madrid to London City Airport recently, from casual conversation behind me it was apparent that one passenger was interlining between London City and Heathrow. He was flying out to the US from Heathrow next morning.

How that happened - whether it was deliberate, accidental or just the only combination of flights available I never found out. Maybe he just upset his travel agent! But getting from London City to Heathrow isn't easy (DLR to Canning Town, Jubilee to Green Park and Piccadilly to Heathrow, if you ever need to know). And he was clearly a naive passenger.

So inter-airport inter-lining does happen: I hope that airport railway station information systems are up to it!