The on-going reliability issues on the Western line with all locos, especially the CSR/BK classes must be giving SCT some headaches. One saving grace is that there are usually other trains around to help out, so delays may be only 12 hours or so. That "luxury" (relatively speaking) may not apply on the MB run, maybe they want to not have the locos running at maximum power for too long, so running an extra loco helps out, as well as providing some back-up for breakdowns. Apart from the Adelaide hills, most of the MP run is over relatively level track with most curves fairly gentle, unlike MB, with steep hills and way too many curves, all demanding high power output, often in short bursts but repeated for days.
That said, 30 wagons might be getting over what one loco can haul, so the second one becomes a necessity. The 50-wagon train would therefore probably need the 3rd loco.
Maybe they can get hold of those LDP class if/when Aurizon ends their hire. Would keep their loco fleet/inventory fairly standard. Then again, anyone know why Aurizon are still hiring those LDP's given the loss of SCT traffic from their trains; I would have expected that some at least would have been de-hired. Maybe their other traffic has built up?