EMD E-units were a line of passenger train diesel locomotives built by the General Motors Electro-Motive Division (EMD) and its predecessor the Electro-Motive Corporation (EMC). Final assembly for all E-units was in La Grange, Illinois. Production ran from May, 1937, to December, 1963. The name E-units refers to the model numbers given to each successive type, which all began with E. The E originally stood for eighteen hundred horsepower, this was the power output of the earliest model, however the letter was kept for later models even though they had greater horsepower.
Like many early passenger locomotives, E-units often used two engines to achieve the rated power. This was due to the limitations of the periods technology limiting the power that could be developed by a single engine. Even so, while E-units were used singly for shorter trains, longer trains tended to need multiple units; many railroads used triple units for long haul passenger service. E-units could be purchased either with or without driving cabs; units with a cab were called A units or lead units, while cabless units are called B units or booster units. B units did contain simple controls for movement purposes, but they weren't meant to be controlled on the main line. The locomotive units were linked together with MU cables which enabled the crew in the lead unit at the front to control the trailing units. Most often railroads would buy either ABA sets (two driving cab-equipped units facing in opposite directions with a booster in between) or ABB sets (a single driving cab with a pair of boosters).
There are still a number of surviving E Series Locomotives including one in the Southeastern Railway Museum in Duluth, GA has Southern Railways #6901 which powered the Southern Crescent.