Within the Sydney metropolitan area, commuter trains are operating among freight trains at 5-15 minute frequencies, though with dedicated freight bypass lines in some places.
One issue in the USA, alongside private ownership of the rail lines, is oversized freight trains and resulting overbuilding required of passenger trains for crash safety. Unfortunately that rules out high-performance EMU designs as used in other countries. I believe Caltrain had to get an exemption for their ongoing electrification upgrades.
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
