If you need to know why Melbourne needs an airport rail link, try catching Skybus in peak hour. It's also expensive - the trip costs four times a regular public transport fare. While that's cheaper than a taxi, the advantage disappears if you're not travelling alone. Despite the faults, it's busy. The big buses will often depart leaving a queue of people who wait for the next one. But there's the paradox: just 7 per cent of passengers travel to Melbourne Airport by Skybus. While on paper it's a high-quality public transport service, it's simply inadequate as the main connection to such a busy airport. And it's not just passengers. There are at least 24,000 workers in the airport precinct, making it one of the busiest employment centres in Melbourne. A staggering 96 per cent of them drive to work. No wonder the traffic is bad.

Theoretically, Skybus could be a lot faster and more reliable by providing it with dedicated bus lanes. But state governments of both stripes have been sadly reluctant to give road space over to buses and trams, no matter what the gains in efficiency, and there's next to no enforcement of the lanes that do exist - just ask any Hoddle Street bus user. Likewise, Skybus could be cheaper, but with demand already swamping the service, in the short term this would cause even more crowding problems. Simply put, buses aren't the mode of choice to serve a destination as large as a major airport. This is why airport rail is important. A single train can move as many people as six or more buses, along a route that doesn't get clogged up with single-occupant cars. Of course if it was that easy, it would have already happened.

Leaving aside the monorail proposal, whose only proponent appears to be a company that constructs monorails, four heavy rail options have been shortlisted: via Sunshine, via Broadmeadows, and two options through the middle. It will be expensive. Given the cost, what will make it successful? The recently opened Southland railway station may point the way. Like the airport, Southland is out in the suburbs, with huge car parks, lots of visitors and workers, and constant traffic congestion. Which is why public transport is an important option. Unless you work there, you probably don't go to Southland every day, yet the new station is almost always busy despite being only months old. It's on a rail line which serves other destinations, meaning frequent service is easily justified. The frequent service means it's easy to interchange if you're not on the same line. The train is also as fast or faster than driving along parallel roads.

The train to Southland serves both staff and customers going to the shopping centre, with a standard fare which is integrated into the broader public transport ticket system, and affordable. These are attributes we should be looking for in our airport rail link, and public transport to the airport more generally: speed, frequency, connectivity, and integrated affordable fares. It should be a reasonably fast trip from the city, competitive with driving. (With Melbourne's growing traffic congestion, this shouldn't be a huge challenge for any competent rail planners.) To maximise usage for travellers from all parts of Melbourne, good connections with short waiting times between trains are vital. And given the airport's significance to the rest of the state, good regional rail connections are also important. Authorities must also ensure that all those airport workers, tens of thousands of them, have viable public transport options. Many of them are likely to live in the suburbs surrounding the airport.