It's not about a single loco being unable to do it. It's about what happens behind the scenes while a single loco is hauling it.
A single WDM3A hauling a train with such a schedule would experience much more wear and tear than any other loco and the repeated notching up and down of the train will also result in an increased rate of failures as it would stress out the internals. A single loco can do the job, but only barely so. Twin charge would greatly reduce the stress on the locomotive. A single WDP4 can do the job, but a twin charge would always provide much better acceleration than a single loco haul, particularly for the longer rakes. Earlier...
more... this year, SCR changed the links of many SF trains running with single WDP4 or WDP4D to twin ALCOs for similar reasons, among others.