The anytrip website shows most train movements around the Sydney area in real-time. If you're not too far away from your desired location, you may be able to figure out what will be coming through. Freight and light engine movements are black dots, suburban/intercity are in colours representing the relevant routings.
However, and this is significant, it doesn't show freight trains on the South Sydney Freight Line operated by the ARTC, only those freight trains using Sydney Trains controlled track.
I'm not sure about trains on the link between Chullora and North Strathfield either.
But it is way better than nothing...
So although Leightonfield is a good place to watch freight trains, they won't show up on Anytrip unless they run on the Sydney Trains tracks (which Minto container shuttles actually do...)
Before the recent reconstruction, Platform 1 at Hornsby gave a good view of southbound trains in the morning. I just don't know what it is like now. Of course you could photograph suburban and NSW Trainlink trains there as well, but not northbound trains leaving platform 5 (which some freights do too).
Mt Kuringai was good for northbound trains in the morning, too.
Canterbury and Dulwich Hill can be good locations for traffic to and from Botany, and coal trains from Port Kembla to Newcastle and vice versa.