Unfortunatley their were only brief notes on the slide stating it was taken at Exeter 09/07/1972 certainly the out of gauge steel loading is ex PK so would have come up from the coast via robo and I would assume that the tanks also did but I guess they could have been added at M.V ?
Not necessarily. Out of gauge trains had fair restrictions on them, & even those off the coast were problematic especially with the single line tunnels & tight loading gauge. I had worked out of gauge trains that came off the SC, from Enfield to Goulburn, & they were regulars on Saturday evenings. They often combined with other loading in Enfield also.
I am not too sure that there were any army bases on the Illawarra that were equipped with Centurians, as most of the lancer barracks were primarily equipped with Light armour rather than heavy.
A couple of other aspects to consider is that usually the loads for 2 engines, including 422cl provided for full loads for the main south, which would have been more than 3 engines on the mountain, also wire transport in those days was primarily from Newcastle, whereas the slab & out of gauge traffic came from the PTK heavy bulk mills.
All that said, it quite possible for the train to have been worked from both PTK as well as from Enfield, with the two combining at MV into one train, also not an uncommon event.