It really depends what your looking for. There is variety in liveries if not loco types on the coal trains west of Maitland, and you have the Cobar/Broken Hill ore trains and northwest container trains along with any grain that is running.
The new road overbridge at Lochinvar provides a fine view, especially for Up trains. You will have plenty of coalies, possibly an empty ore train, and depending on the day a Down Freightliner intermodal towards Narrabri mid- to late-afternoon.
Further north, the road overbridge at Ardglen, also provides good views in both directions. For the Up Freightliner intermodal out of Narrabri, you might need to be up that far, as the master train plan has it through there at 1603, and if it is held up anywhere, the light might be gone further south. The QUBE intermodals are night runners in both directions.
Closer to Newcastle, the Crawfords train out of Sandgate is about to lose its C class power, as new locos are outshopped from Goninans, so that may be an opportunity for you to get the last of those.
Have a look at the new MTP which is to commence on 6 March. That may help you decide which days to venture out.