A possible day one MM1/Airport/Melton operational plan
Pakenham-Sunbury & vv. Pakenham SAS to Caulfield, express to Anzac SAS to Footscray express to Sunshine, SAS to Sunshine via Airport
Cranbourne-Melton & vv. Cranbourne SAS to Caulfield, express to Anzac, SAS to Footscray, express to Sunshine, SAS to Melton.
Watergardens-Westall & vv. Westall SAS to Caulfield, express to Anzac, SAS to Watergardens via Footscray/Sunshine.
West Footscray-Pakenham or Cranbourne & vv.. West Footscray express to Footscray, SAS to Anzac, express to Caulfield then SAS to Pakenham or Cranbourne.
Westall-Sunbury & vv. Westall SAS to Caulfield, express to Anzac, SAS to Sunbury via Footscray/Sunshine/Watergardens
4TPH Pakenham-Sunbury (where airport is serviced). This is a shift from 3TPH to 4TPH for the Pakenham side and shift from 1.5 TPH to 4TPH for the Sunbury side.
4TPH Cranbourne-Melton. This is a shift from 3TPH to 4TPH for the Cranbourne side and shift from ~2TPH to 4TPH on the Melton side.
4TPH Watergardens-Westall. This is a shift of 3TPH to 4TPH for stations between Watergardens to Sunshine). And a smeg more for the city-westall side.
12 TPH between Sunshine-Westall.
6 TPH Pakenham-Sunbury (airport is serviced). This is pretty much a near match - train-for-train - on both Pakenham/Sunbury branch peak timetables.
6 TPH Cranbourne-Melton.
6 TPH Watergardens-Westall
18TPH between Sunshine-Westall
Given it's 5 years away there'll be some growth to Watergardens and then some also around Sunbury (there's a large green wedge between Watergardens and Sunbury - Watergardens/Sydenham area is pretty much built out - Diggers orphaned with the Urban Growth Boundary) we might need to add extra trains (don't forget, like-for-like, the trains get bigger in 2025 for Sunbury and definitely Melton side because of differences from Siemens/Comeng -> HCMT and Vlo -> HCMT).
Slot theses in where needed over time before more trackages is needed:
2 TPH West Footscray-Pakenham. Pakenham has roughly 6TPH in peak now - this is a new service.
2 TPH West Footscray-Cranbourne. Ditto from Pakenham remarks above.
2 TPH Westall-Sunbury. Adding more Sunbury capacity but also linking Albion-Ginifer-St Albans-Keilor Plains-Watergardesn with Diggers and Sunbury (which I assume will have very few start/end journeys among those stations).
18TPH + 6TPH = 24TPH between Sunshine-Westall.
Airport gets a bare minimum 4 TPH metro + 1-2 TPH Bendigo regional from day 1 in this scenario too.
Edit x 2: for an overview of the 'spend whatever's left on an Airport-Diggers alignment' from post above this one, [size=3]https://www.google.com/maps/d/viewer?mid=1k-v4Ucx0_1HErzk0JBCH-3IzU_EQ9aTW&ll=-37.70972174944803%2C144.80138940000006&z=13[/size]
Original context of that map from a discussion on SSC - see quoted part [size=3]https://www.skyscrapercity.com/showthread.php?p=165243896#post165243896[/size]
edit x2,345.034: Jesus H Christ, can the admins please fix external links!