Look, I understand the attraction. Burra is a pretty town and its importance to heritage preservation can't be overstated
. Riverton and Saddleworth combined are the next reasonably large settlements along the line. However...
Tarlee to Burra would be ~75km by rail. That's a lot of track to maintain. The proven ideal track length for a heritage railway is 10-20km (making for a one-way trip time of 45-60 minutes). Steamranger is about the same length all-up, but they only run trains along that full length for a fraction of the year. You'd be looking at a lot of dough to rebuild 75km of track - and that's before you even start thinking about running an actual train on it! Unfortunately there's not much but long stretches of farmland between Burra and the next town of any note (Manoora or Saddleworth, depending on how charitable you're feeling). The only stretch that might
be remotely viable would be Riverton to Saddleworth (~9km) - nowhere near Burra.
While no doubt needing plenty of work the track didn't look to be in terrible condition where I saw it. Initial cleanup of the corridor would not require expense - just, as you say, many dedicated volunteers. In the age of social media, such a concept has the potential to pick up momentum more easily than previously. I agree there would be plenty of considerable cost to actually get heritage trains running on the line, you have to start somewhere!
Track condition is like an iceberg - what lies beneath the surface is much more important.
The rails themselves might be in fair condition - but all of the joints will have been rusted 'frozen' by now. At a minimum you're looking at servicing every second or third joint just to keep your track from buckling. Then you look at the fasteners. Dogspikes can be deceptive as they can look affixed but are simply just rusted into the baseplates, with wallowed-out holes in the timber sleepers that they have been driven into. The sleepers themselves can look fine on the surface but are in reality eaten out by termites from underneath. I have seen this many, many times. I can also guarantee a ballast deficiency - and the leftover rock won't be much chop either.
Then comes the perway. Any timber bridges along the line will need wholesale replacement. Drainage is critical; cuttings along the line will be totally overgrown and all of the cess drains will be clogged. That's a lot of civil work that isn't going to be able to be done with volunteer labour alone. And don't get me started about recommissioning all of those level crossings.
To synthesise this: everything below the foot of the rail would need to be removed and replaced. 100% steel sleepers with resilient fastenings (aka Trak-Lok or Pandrol clips) on fresh ballast would be the most economical track structure to use. Tens of millions of dollars would need to be spent on materials alone, let alone contractor hire or plant and equipment for volunteers to use.
You want another heritage railway in SA? Look at the Barossa Valley.