The class E motors are the reinvention of the class B motors, taking advantage of 45 years of improvements in electric traction to fit more speed & horsepower into the compact centipede shape of the original class B’s.

The class D locomotives were a gratifyingly large step up from the 4000 hp, but not much speed, that the class Bs provided, so when they were sold the motive power department did some some research into higher powered machines, including having the Portland shops build up a few prototypes. The B1x was a little too much (an awful lot of body segments & trucks, which had the potential of being a maintenance nightmare that could overbalance the terrifyingly huge power output of the thing) but the B4s that were outshopped in 1967 threaded that needle; the normal class B wheel arrangement and dimensions, but over 50% more power (the B4’s came out of the shops producing 7400 HP, but were really slippery, so they were downrated to 6100 HP and that stopped their ice skating tendencies), so as a result the railroad decided that instead of going with GE traction (no more Baldwin, alas, and GM was working on a much smaller scale than what the PV&T wanted) they’d let the Portland shops do a production run of what they’d learned from the B4’s.