I just read what many are reporting as the "World's busiest routes" (including our own Alex).
It sounded strange to me that Hong Kong to Taipei would top the list. Almost 5m passengers is impressive, but… double that and you're getting closer to reality.
Although the numbers are not the freshest, here are Amadeus' numbers for 2012 (rounded to closest thousand):
1. Jeju - Seoul — 10,156,000
2. Sapporo - Tokyo — 8,211,000
3. Rio de Janeiro - Sao Paulo — 7,716,000
4. Beijing - Shanghai — 7,246,000
5. Melbourne - Sydney — 6,943,000
6. Osaka - Tokyo — 6,744,000
7. Fukuoka - Tokyo — 6,640,000
8. Hong Kong - Taipei — 5,513,000
9. Okinawa - Tokyo — 4,584,000
10. Cape Town - Johannesburg — 4,407,000
See? The HKG - TPE leading the IATA chart is only number 8 (and has seemingly seen a decline since 2012).
To be fair, the list Alex linked to should have probably been named "World's busiest international routes"—as you realized above, most of the busiest routes are actually domestic, with a concentration in Asia.
In the ranking above Tokyo really means Haneda. It's a domestic airport. And it's the fourth busiest airport in the world!! (some international routes have been restarted since 2010, but are still negligible)
Another way to visualize it: one usually associates 747s or 777s with long-haul travels. Guess what? I used to travel HND - CTS in a 747. They were a common sight (were, as they're being phased out). JAL used to have economy-only 747s for the Osaka - Tokyo route, that's 500 passengers.
That Osaka - Tokyo is actually the world's busiest commute: according to some reports, the Osaka - Tokyo train route sees …120,000 passengers per day—that's more than 40 million passenger a year!
40 million passengers a year. Let it sink in.
A train every 10 minutes.
Let me try to talk to you in plane terms: that would equal to more than …650 economy-only Boeing 737 MAX 8 aircrafts.
No Japanese airline can afford that.
Back to the numbers with one further remark: they also undercount O&D passengers, meaning passengers who transit via an airport that wasn't their point of origin (O means origin, D destination and that's how airlines count passengers). Some argue that Tokyo - Sapporo still tops the ranking with that in mind (the goal here is not to have a petty battle around who's #1, but to realize how huge these domestic lines are).
I hope Amadeus releases some up-to-date numbers soon. I'm particularly curious to learn about the rising routes in China. Here are the 2011 numbers (rounded):
Beijing - Shanghai — 6,800,000
Shanghai - Shenzhen — 3,800,000
Beijing - Chengdu — 3,500,000
Beijing - Guangzhou — 3,300,000
I have a feeling that the top 10 will be soon be disrupted.