Does it matter which airport you land at?

Japan

By about an hour, yes. Haneda sits inside Tokyo; Narita is in the next prefecture.

China

Yes. Capital sits inside the city in Chaoyang; Daxing is far to the south with its own separate railway. They are not interchangeable.

Thailand

Yes. Full-service airlines use Suvarnabhumi; low-cost carriers mostly use Don Mueang, 46km away with no rail link of its own.

Vietnam

More here than almost anywhere. The three international gateways are not alternatives to one another — they are the north, the centre and the south of a country the train needs 36 hours to cross.

South Korea

Gimpo is far closer to Seoul, but almost every long-haul flight uses Incheon, so you rarely get to choose.

Taiwan

Yes, pleasantly. Taoyuan is the main international gateway outside the city; Songshan sits inside Taipei itself and handles domestic plus a handful of regional routes.