Only because the Wakefield's loading is so large at the moment, and the 1200m maximum length trains, PeaCo can only get a few wagons on the 7901, so PN run the extra 7903. With the good harvest last year and looking good for this year here's hoping for more trains.
There's also extra loading coming out of Merbein so 7903/4V take the Donald load as well as the extra load that couldn't be accomodated onto 7901/2V.
for a couple months recently they were getting by running separate trains to Donald as 7907/8V but once the loading picked up a bit more at Merbein that stopped.
As a Mildura resident there is currently 10 trains a week heading up to Merbein. These are my observations of the current arrangement -
Tuesday: Down 7901V arrives usually in the very early afternoon. Departs as 7902V at around 2300hrs (timetabled to leave at 2130hrs but it leaves later than timetabled to facilitate a cross at Maryborough. Once 7903/4V is no longer needed i would say it would go back to the normal timetable)
Wednesday: Down 7903V arrives usually in the early afternoon. Departs as 7904V at around the same time? (I'm not sure).
Thursday: Down 7901V arrives in the early afternoon. Departs as 7902V at around 2300hrs.
Friday: Down 7903V arrives in the early to mid afternoon. unloads at Merbein then heads to stable at Yelta.
Saturday: Down 7901V arrives in the early to mid afternoon. Unloads and stables overnight.
Sunday: Up 7902V departs Merbein usually around 1600hrs.
Monday: up 7903V departs Merbein usually at around 2045hrs.