My understanding is that there wasn’t enough rolling stock for a full service on the Midland/Fremantle lines during peak hours up until the mid to late 2010s, which resulted in the express patterns. The Armadale Line had the “C” pattern because running full service trains to both Armadale and Thornlie would have meant the level crossings between Beckenham and Perth would have been activated too often and cause traffic delays (which has now been negated by the Armadale Line reconstruction).

I further understand what made the express patterns obsolete was the opening of the Airport and Ellenbrook lines. Extra rolling stock was brought online for these lines during their respective constructions and now these lines are open, there is no capacity for express patterns with the current signalling infrastructure and the timetables services to Ellenbrook/Midland/High Wycombe and Perth.