GSR wants to get the run down under 10 hours, preferable 8 hours. You'd
need 160km/h running for long stretches to do that.
The sensible step would be to order 2 more XPT sets (plus spares) for
Countrylink, and then for GSR to lease them for the Melbourne - Adelaide
run. Each arrival in Melbourne from Sydney could then run on to Adelaide,
and each arrival from Adelaide can run on to Sydney. The interworking
would be very simple.
Cheers
David