When did this whole Mickey mouse DPU ever come about? What ever happend to Power on the Point anymore?
DPU allows bigger trains with lower in-train forces.
Think of a DPU as a constant helper. Most knuckles break at 263# lbs of force if I remember right. By putting one engine in DPU on the rear you get lower forces and also less slack action since the rear engine keeps the slack bunched up plus faster sets and releases of the brakes. You are getting air from both ends and recharges the line faster.