AFAIK there a blackout periods for freight on the Sydney Trains network during the morning and afternoon peaks, though.

Plus the "dedicated freight bypass lines in places" nowadays is basically the complete route between the southern limit of the suburban rail network at Macarthur and Port Botany via Enfield Yard, plus a stub from Enfield towards North Strathfield. So major track sharing (especially with super-long interstate freight trains) only really happens from Strathfield on the line towards Newcastle, and that again is at least three or even four-tracked for parts of the way within Sydney (although unlike the southern half it's not exclusive freight infrastructure).

Plus whatever local-ish freight traffic might still exist around Western Sydney, towards the Blue Mountains, and along the coast to Wollongong.

Longer trains are more efficient. That is why we run them