Despite the best-laid plans, when it's time to get a train out of town they'll grab whatever units they can get. It's like when Santa Fe first got their "Superfleet" Warbonnet units; it was decreed they only be used in like sets on high-priority trains. In short order, I was seeing them as helpers out of Bakersfield. "Got to get that train out of here, and that's the only power in town." Also, ever notice that work trains usually seem to get big, new power?