Autopia notes the latest rankings of the world's busiest airline routes. Sydney-Melbourne comes in fourth (with 1.5 million passenger last year), though maybe it will move up a notch this year. Hong Kong-Taipei is currently first on the list, but China and Taiwan recently opened direct air passenger links. Previously, passengers had to go via Hong Kong or Macau to travel between Taiwan and China, but no longer.
In its commentary on the list, Autopia notes that many of the routes are short and could easily be replaced with or supplemented by high-speed rail. But Autopia specifically excludes Melbourne-Sydney from this consideration, saying it 'doesn't make much sense' to travel by rail between them.
That certainly seems to be the attitude in Australia, but it's worth pointing out that China is now building a 1318km high-speed rail link between Beijing and Shanghai (by comparison, the driving distance between Melbourne and Sydney is about 900km).
The Beijing-Shanghai line will reduce the train travel time from ten hours to less than five. The flight for this route takes just under two hours, though when you add on the transit times to and from out-of-town airports and the greater comfort of rail, the train option still looks competitive. It's greener too.
Photo by Flickr user kamoda, used under a Creative Commons license.