Most commonly used air routes, in million passengers a year
http://maps.grida.no/go/graphic/most-commonly-used-air-routes-in-million-passengers-a-year

Osaka
Sapporo
Tokyo
Hiroshima
Fukuoka
Kanazawa
Kagoshima
Seoul
Naha
Beijing Hong Kong
Shanghai Guangzhou
Los Angeles
Edinburgh
Chicago
London
Atlanta Glasgow
Amsterdam
Dublin
New York
Milan
Paris
Orlando
Toulouse
Rome
Fort Lauderdale
Madrid
Nice
Barcelona

Flights most easily replaced by train (short distance, no sea to cross*)
Flights possibly replaceable by train and ship (short distance, sea to cross)

Other options
In both cases, ship and train replacement options need to be politically supported to become or besides flight? remain attractive (schedules, prices, comfort level).

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