Did a bit more research and this is a Whitton era thing, and note that Gunnedah was built in that era. Whitton rarely had the funds to duplicate, but planned and built with future expansion in mind.
If you look at major locations where the station was provided on the Up side, the “Through and Platform” layout was adopted. Some examples:
Singleton, Scone, Murrurundi, Tamworth, Armidale, Glen Innes, Tenterfield, Gunnedah, Wallerawang, Mittagong, Cootamundra, Wagga Wagga, Gundagai, Jerilderie.
Where a station was built on the down side the “Main and Loop” arrangement seems to have been preferred, eg Aberdeen, Quirindi, Moss Vale, Gunning, Narrandera, many others.
I’m struggling to find any instances of “Through and Platform” locations with the station on the Down side. Any suggestions?
Not sure what you mean by the last sentence, are you looking at locations without a loop or other sidings, as you would find quite a few like that also with the through line or main at a huge proportion of locations as well.