I have just returned from yet another rail trip across USA.
There are significant differences between USA "short lines" and rump of the old SAR.
Typically short lines have:
A continuum of traffic.
A relationship with the end user be it food processing plant, steel mill, factory, textile mill.
Minimal operations staff. (family owned & operated)
Minimum legislative compliances.
A major railroad system to feed into.
A short route.
One of the closest examples of short line operation we had in Australia was the Silverton Tramway; it did what the government systems could not or would not do.
Considering that the USA is the home of the automotive industry it never ceases to surprise me how much freight moves by rail and how few trucks there are on the highways.
The USA mainland is roughly size of Australia with more than 10 x the population and probably 20x the industrialisation, ie they make things, not just dig holes in the ground. Therefore they have the traffic density to sustain rail lines where Australia doesn't. ie if Wagga Wagga was 5m people, Dubbo 2-3millon, Can 2-3m, Albury 1-2m, Moree, Tamworth, Armidale etc etc. its likely the number trucks on the roads may not increase a great deal. There would be viable, efficient rail lines criss crossing the country side with multiple operators.
Most of the USA population is inland, unlike Australia, so this also helps rail be more viable.
Also if you have a factory on east coast of USA and want to ship to any state capital in Australia, there is a good chance the freight will go via rail to the west coast of USA then ship to the relevant capital and likely in vice versa.
If Tasmania had only one major port, Tasrail would not be at best marginal operation.