I took it to Melbourne last Friday - the first Overland of 2020. There were 40-50 pax, not too bad given we all had less than a week's notice it was returning. I had to cancel a non-refundable flight to catch it, but I was coming from Perth on the Indian Pacific so I wanted to complete the entire journey home by rail if I could - 3,497km!
We left Parklands Terminal on time, arrived ~15mins early into Murray Bridge (but left on time), and were 10mins early into Southern Cross. It definitely feels fast, there's not much time spent travelling slowly, or waiting for a signal.
There does seem to be a few carriages that are surplus to requirements. Consist was:
Red car: blocked off with 'staff only' signs. Same thing last time I caught it to Adelaide 2 years ago, when is it ever used?
Red car: with ~1/4 of the seats reserved for V/Line customers
Red premium car
Why drag the third Red car all the way from Parklands terminal to Melbourne and back, when it's guaranteed it won't be needed, for this trip at least?
And given there was only about 10 people with checked baggage (myself included), do they really need a separate baggage car? Can't fit in the power car, or put them in the empty Red car (which would have had AC, unlike the baggage car!)?
Re additional carriages for increased frequency: I did notice many many spare carriages at Parklands, some from The Ghan, some without signboards, probably many undergoing maintenance/refurbishment, and probably some are only surplus during the Great Southern season, and perhaps they're all sleepers anyway.
The trip itself was excellent, the staff fantastic (one of them was on my Indian Pacific trip as well, so the same high level of service). On board food (and beverages) great. Imagine if the XPT had this level of service!
But I agree with others here JBRE hardly advertises it, and there was no Overland merchandise in the Parklands Terminal gift shop at all. I did get a pen & mug on board (so not sure why they don't sell them in their shop).