I think this train number is the service from Broken Hill to Port Pirie but can someone please confirm.

By no means sure now but trains from the HIll to Pirie used to carry even numbers.
IIRC the down ore train was xx55 and the up ore was x100.
But not sure - nothing would surprise me these days.

I think this train number is the service from Broken Hill to Port Pirie but can someone please confirm.

The numerical area codes are defined in the ARTC TOC manual...

So any train from Broken Hill to Port Pirie will be 54xx, the reverse 45xx

On 12 January (if I recall correctly) I photographed 4500S which was the empty concentrate train from Pirie to Broken Hill

The suffix "S" is said to refer to trains that don't leave South Australia, which is patently incorrect for any train between Broken Hill and Port Pirie. It might refer to trains running on former SAR tracks, which did reach Broken Hill.

The Master Train Plan should show that train and who operates it.

M636C

I think this train number is the service from Broken Hill to Port Pirie but can someone please confirm.

The numerical area codes are defined in the ARTC TOC manual...

So any train from Broken Hill to Port Pirie will be 54xx, the reverse 45xx

On 12 January (if I recall correctly) I photographed 4500S which was the empty concentrate train from Pirie to Broken Hill

The suffix "S" is said to refer to trains that don't leave South Australia, which is patently incorrect for any train between Broken Hill and Port Pirie. It might refer to trains running on former SAR tracks, which did reach Broken Hill.

The Master Train Plan should show that train and who operates it.

M636C

Not quite. This train runs to Perilya siding, which is in SA territory. SA territory for rail extends to signal 56 which is at the SA end of Broken Hill yard just for a bit of extra info.

5455s runs loaded from Perilya Siding to Port Pirie, returning 4500s.

The Route Access Standard (RAS) has the detailed information on how train numbering works

http://www.artc.com.au/library/Train_Numbering.pdf