I'm also interested in the differences between the divisions. They're sometimes mentioned in the various SAR books and the Convention notes but the distinction is unclear.
Were they basically operational assignment areas for different locos and trains, and otherwise there wasn't a great deal of difference between them? Were locos regularly changed when they crossed division points (e.g. 500s took the Overland over the hills, which were then changed to 600s - did something similar happen with freights?). I guess coming in from the narrow gauge it's obvious that loads were transferred and engines changed, but how about elsewhere?n
Most steam locos were as you say, changed at the division point which was Tailem Bend, conveniently at the extent of the range of the engines coal and water resources. 7 of the 600's did handle most if not all of the main work from TB on east while 3 were allocated to Adelaide Division. All 10 would most likely have been allocated to TB in their last years.
I don't know of the allocation of 700's/720's or 750's if any were actually assigned to TB depot although I have seen a picture of a 700 at Renmark. I believe that there were two 620's allocated to TB in the early 60's and 624 ran only one extra wheat special to Karoonda in 64 as the 830's took over most steam workings very early proving their reliability, so 624 (and 627?) were withdrawn to Adelaide circa 1965. I don't know how often a 500 would have gone to Serviceton and the 500 book states that it was highly unlikely one ever went to Mt Gambier.
Apart from the 830's and 500's at TB depot and the 2 BG 500's at Peterborough, most engines worked through and the division points became more of a manpower allocation rather than locomotive ones.
Peterborough and TB crews often worked to Adelaide as did some Pt Pirie crews on selected runs to Adelaide and Peterborough, although the Pirie crews mainly worked shunter jobs on either Broad or Standard Gauge and swap jobs where they would meet an Adelaide crew on a pass or freight and swap trains. Peterborough crews in the early days of the BG had one of their destinations as being Riverton where the barracks were part of the station building ( and you thought it was a bigger building because it was the refreshment rooms stop...) and also did swap jobs with Adelaide crews. as well as going through. Most Peterborough Drivers were Bluebird qualified so a usual turn was to take a freight to Adelaide and drive the Bluebird back, the fireman travelling Passenger.
AFAIK, Mt Gambier and Tailem Bend crews also did regular "swap jobs" as did possibly Renmark and TB crews. And of course Adelaide Bluebird drivers swapped at Keith with Mt Gambier drivers, the crib break being taken at Murray Bridge in both directions...
Edit - Also AFAIK to emphasise the crossover of division points, the Cambrai/Sedan Goods were operated by TB crews who drove their 830 to Monarto South then North. Whether wheat was left at Monarto South to be picked up to go to Pt Adelaide I am not sure!
Hope this Helps