The Kensington grain usually only runs on Mon/Wed/Fri, normally it arrives into the siding at daybreak and departs sometime around 1400-1530.
I understand that if a path can't be obtained prior to 1530 it has to wait until after the PM peak i.e. 1830.
I was lucky to score the H-BL combo at Kensington, I just happened to go past North Melbourne on a Werribee bound spark when I spotted the train stationary behind platform 6 waiting at the home signal to access the Coburg goods line, I bailed out of the spark at South Kensington and ran to Kensington Station not really knowing whether I'd make it or not. Luckily Metro were running a few sparks out to the Showgrounds that afternoon which delayed the grain transfer a little, I probably waited about 20 mins for it to show up.