Artist's impression of the City interchange for the proposed Canberra light rail.

Artist's impression of the City interchange for the proposed Canberra light rail.

The suggestion of a light rail system for Canberra has me worried, and I'll tell you why. I bet the Greens will insist that the trees be kept on Northbourne Avenue, with the result that there will be room for only a single line.

They will point out how much money this will save and that it would be a standard application of queueing theory to organise the number and placement of sidings. Moreover, to clinch their argument, they will point out that this kind of thing is common in Australia.

I can feel a digression coming on here. Did you know that for much of the journey between Sydney and Melbourne, there is only one line of track, so that trains travel in opposite directions on the same line at the same time? And this has been going on for years.

I used to think that when a train leaves Melbourne the line is kept clear until it gets to Sydney, when another train can travel in the opposite direction. Apparently not. If they did that, they could get only one train through in each direction every day and there might as well be a sea route, with the likelihood of periodic massive disruption by wharfies.

The truth is that trains leave Sydney even though there is a train bearing down on them from Melbourne with barrels of Foster's for thirsty NSW and with crowds of AFL fans, who will be getting off in Campbelltown, happy that there is still one Sydney team they can beat. Somewhere along the way, someone decides to shunt one of the trains into a siding in order to let the train in the opposite direction pass.

Have you any idea of the possibility for error in this? The guy could be asleep, or he might be new to the job, or hungover. Apparently, the whole process is co-ordinated from Junee. That's Junee, as in the location of a private enterprise jail. Suppose there is a jail breakout and the escapees decide to take over the only thing in Junee that is worth taking over apart from the Laurie Daley family pub - the railway station.

How do they know what levers to pull so that one or other of the trains can go off into a siding? There would be no point asking whoever normally does the job because any good 1960s film would tell you that he would be the first person to be knocked out and everyone else would be tied and gagged and put in a locked room. So the baddies would have to make the decision and they, of course, would be fighting among themselves, especially if Charles Bronson was playing one of the characters.

All of which gives me a great idea for a film. It could be called Southern Aurora and would star Leonardo di Caprio as a footballer home for the weekend, and Kate Winslet as the squeeze of a former media mogul. There could be lots of shots of red blinking lights approaching each other along a wall panel. And there would be a clock counting down to the moment when the trains pass through Junee and as the second hand approached the dreaded hour and a train whistle was heard in the distance, the muscled hero would be cursing his wonky shoulder (cut to flashback involving Barry Hall) as he frantically tried to pull the lever to send one of the trains into a siding.

Normally he would succeed, but in my film he would not and there would be massive carnage followed by 40 minutes of the hero and heroine wandering through idyllic fields declaring their undying love. Obviously that part couldn't be set in Junee, what with the absence of fields, idyllic or otherwise.

Anyway, to come back to the single line of railway. You walk up to a booking window at Central Station in Sydney.

''Return to Melbourne, please.''

''That'll be $311.60. You realise, of course, that a train left Melbourne 20 minutes ago and will be travelling on the same line as your train.''

''But …''

''No need to worry. There's a guy in Junee who will make sure that one of the trains pulls off somewhere to allow the other one through.''

''But suppose he forgets.''

''It hasn't happened yet. There's nothing to worry about. You can trust the NSW Railways.''

You curse yourself for not holding on to your Flybuys and get on that train as near to the back as you can.

OK, all of that is a long way from the LRT - I did warn you it was a digression - but you can imagine a similar scenario on a journey from Gungahlin to Civic, can't you?

Anyway, that's what I think.

Frank O'Shea is a Melbourne writer.