DPU is an acronym for "Distributed Power Unit" meaning a single locomotive operating in "distributed power" mode. "Distributed Power" is just that -- the power is distributed throughout the length of the train instead of all concentrated on the head end. All locomotives in the consist (wherever they may be in the train) are operated from the head end using one crew. DPU's do not necessarily always go at the end of trains, though this often happens as it is easiest when adding or removing power at a terminal. The actual placement ofthe additional units depends on the tonnage profile of the train and the terrain over which it will operate. In years past, this same concept was accomplished using manned helpers for a short stretch of a particular territory. Today, DPUs usually run through with a train over greater distances, sometimes never coming off the rear end.