Tony et al
The proposed timetables as described are wishful thinking at best.
Do the graph!
The semi express takes 35 minutes from Seaford, the stops all to Woodlands Park takes 44 minutes; allowing 2 minutes headway at either end equates to 11 minutes between trains if a stops all to WP is interleaved between the semi expresses.
The aproximate 7.5 minute frequency may work if trains run express to/from Brighton then stop all to/from Noarlunga Centre and the Stoppers turn back at Noarlunga Centre.
Mathematically the difference in departure times of trains cannot be less than the journey time difference between the fastest and slowest train plus two times headway. This can be improved if one train can overtake another but on double track requires one train to run bang road.
I can't see how they can make those times at all, let alone maintain that frequency. Currently an express from Norlunga to Adelaide takes 32 minutes. I cant see how a semi express can travel an extra 6km's stop several more times and only add three minutes for the trip.
Furthermore, an all-stops to woodlands park from NC would take 44 minutes. Perhaps with sparks the improved acceleration would help and trim of some of the time on an all stops, but I can't see how it would work with the semi-express.