As far back as the 1960's, I saw proposals for Hi-Rail buses. A couple were actually produced - I seem to recall a short item in Trains about NY wanting to use them on a disused LIRR line to do an express to Kennedy airport. Problems of course included cost, maintenance, and a very rough ride. Not to mention lack of FRA compliance which nobody cared about at the time. Seems like an item for a very small niche market - low-density areas where essentially abandoned rail lines exist for the "main line" part of the bus run that could be patched up fairly cheaply, and where parallel roads either don't exist or are extremely congested. If the rail line is in fact totally unused otherwise, it might be cheaper and more comfortable to pave it over and use regular buses on it.

According to the Sunday paper, exchange rate last week was about Aus$1.25 per US$1.