If you're thinking into the future for sparks to Wallan, then consider what the network will look like then.Why on earth would you want to ride a shaky Xtrap 48km (hanging on the straps homeward), when you already have a reasonably comfortable Vlo. @ptv you ride Frankston don't you, Wallan is worse.Seymour rarely sees Velocity trains, most of them are sprinters or the good old (not really, just old) N sets, sometimes with H class carriages on them for good measure. I'd take a metro train built this century over those any day.
Flinders St to Craigieburn via the loop is 47mins (would be much the same via Upfield). VLine is timetabled from Craigieburn to Wallan in 14mins off peak with one stop. Given the metro trains can accelerate much faster this would probably be about the same, but lets call it 15 to be fair.
So Flinders St to Wallan via the loop is 62mins, just over an hour. That's pretty much the same as Lilydale and Mernda for comparison (gee, shows how painfully slow the trip to Mernda is given the distance - Wallan is much further out!).
Not too big an issue after all?
V/Line expects to replace its entire locomotive-hauled fleet with VLocity units by 2022PS: "good old (not really, just old) N sets" are mostly the Shepparton train, also slotted to become Vlo.
Accelaration by sparks is undoubtedly better, but only relevant when you consider how many stops.
If you think the Wallan train would run express for the most part (like today), there is little advantage.
Your time comparisons perplex me "Not too big an issue after all?"
So you argument is to replace a Vline service (~60 mjnutes) with a Metro service (~60 minutes).
Spend another squillion dollars to deliver NO benefit.
Both Craigeburn and Upfield are haunted by 2 track only, and dozens of Level Crossings. Future prospects for increase in speed and/or frequency are bleak. Lilydale, Mernda, Frankston etc... at least have plans underway to make them better.
Repeating myself, divert via the Airport holds the answer to huge improvement that sparks to Wallan does not.