Often times the diesels are there to provide dynamic braking (on runs with heavy grades) and as protection power in case anything happens with the steamer, so that the train can get cleared up into a siding and not foul a busy main.
Larger steamers like the UP's duo and NS's now cold 611 and 1218 had/have no trouble with 15-20 car consists under most circumstances.
In the early 1990's (can't remember which year), Atlanta & West Point 290 (a heavy 4-6-2) subbed for NS's 611 & 1218, neither of which were ready for an early NS trip out of Jacksonville. The 290 came with a pair of New Georgia F units behind it. The steam crew commented that the (I think) 18 car consist was no trouble for the 290, aside from needing a short boost from the F's to get it rolling, but that it kept it at speed all on its own.
Wondering if I'll see the Western Route double-tracking finished before I retire...
Photo: Melbourne W7 No. 1019 on Route 78, Bridge & Church Streets, Richmond, Victoria. 10/21/2010