The same track takes twice the number of services during peak hour, so that surely can't have been a problem. Even on the old signalling system they never used to have any problems running a near-full peak service for both Noarlunga and Belair on one pair of tracks during the week of the Royal Show.
I'm leaning towards the signalling being incomplete, a bit like the Elizabeth turnback fiasco.
I also suspect the signalling is only half complete & maybe it's not possible to signal trains between the Noarlunga & Belair lines at Goodwood at the moment.
The Seaford lines are eventually meant to have bi-directional signaling through this section. While the "wrong direction" signal heads are installed on the gantries, these are not yet illuminated.
Also most of the new switches & crossings between the Noarlunga and Belair lines are heavily rusted and even have small areas of paint remaining on the railhead from time of installation, which suggests they have never been used. The exception is the down to up crossover on the Noarlunga lines, which was used to reverse the Show Shuttle trains for a week or so earlier this year.
"Why can't the Belair trains run on the Noarlunga lines from Goodwood?" - you could ask the same question about the Night & Weekend closures of the Belair line, which have quietly sneaked back in until 31st December. Now that the gantries are all installed, could the electrification contractors not work on one pair of lines at a time so the Belair trains could keep running a normal schedule? Yeah, yeah, I know it's Health & Safety
, but there is only one railcar per hour to Belair during these times, so surely a bit of supervision & a TSR for trains through the work area would not be too risky.