This picture was taken near Diapur Loop, in far western Victoria on the main train line between Melbourne and Adelaide. I spent about 15 minutes sitting here yesterday, waiting for a freight train to pass in the opposite direction. During that entire period, my Next G modem couldn't scare up a signal at all, despite repeated attempts. My BlackBerry using Optus managed a 3G signal throughout without problems. (I'd hoped to test Vodafone as well, but the credit on my test device SIM ran out. Gah!)
Next G worked fine on the Overland in other locations, so I don't think this was a case of the train causing problems. It's largely a reminder that figures about covering a given percentage of the country never tell the whole story: the only detail that really matters is whether you can get signal when you need it.