I expect that will be deferred indefinitely though, since it's clearly not essential at this time and money is a major issue at this time. The DPTI's own submission to Parliament for the grade separation project included a non-specific mention of an upgrade to Keswick that was to be deferred until later, but I would not be surprised if it dies completely, considering the tight space available and the expensive need to build it to full DDA standards.
Done right, the crossovers north of Leader St should be able to work for continued use of something similar to the current Showgrounds Central routine (i.e. all regular trains running through platforms 3-4 at Keswick, Show shuttles through 1-2) as well as being exactly the same layout needed if the "overtaking" routine with peak stopping/express trains continued. I'm not in favour of having trains overtake each other on that section*, not when an all-stops train only loses 5-6 minutes over an express to Woodlands Park.
Even with the current Keswick station though, I think it would be better to serve the Show by dumping the Showgrounds Central platform and running a special event timetable using the Belair line - two trains per hour through to Belair with strengthened consists and two shuttles stopping at Keswick before terminating at Goodwood. No gate to the Showgrounds is too far away from public transport then, not when you add together Goodwood station (south-western corner), Keswick station (north-western corner), the purpose-built bus bays in front of the Goyder Pavilion and the normal bus stops on Leader Street and Goodwood Road.
* having a Seaford stopping train wait at Brighton platform 2/3 for an express to overtake could work well though!
Do you or any other know what the planned arrangement for crosssovers north of Goodwood looks like
There has been no problem with overtaking trains in Adelaide, it is done world wide. The few minutes gained will be important for the new Seaford time tables which are supposed to have increased frequency. Whethe it is effective when only two stops are involved is a question.
I agree regarding overtaking at Brighton. If you do a train graph of the present NC timetable you will see how much the expresses jamb up the number of trains which can run. Having the express stop at Brighton and also overtake a stopper allows a higher frequency.
If the Seaford line can access both South Sub and Main north of Goodwood then Showgrounds trains could still use a temporary platform; the trailing cross over that was on the sub would need to be put on the main for turnback.
I never knew if the train to the showgrounds got much use.