The EMC AB6 was a type of diesel locomotive built exclusively for the Chicago, Rock Island and Pacific Railroad (the "Rock Island Line") by General Motors' Electro-Motive Corporation and delivered in June 1940. Two examples were built, numbered #750 and #751. They were built for the Rocky Mountain Rocketpassenger train, which travelled as a unified train from Chicago, Illinois, to Limon, Colorado, which then divided. One section went to Colorado Springs, Colorado, and the other to Denver, Colorado. The Rock Island desired a locomotive that could look like an integrated part of the train during the Chicago-Limon portion of the route, and could then be operated independently to take three cars to Colorado Springs. A regular, cab-equipped A-unit could have been purchased, but that would have ruined the streamlined look of the train, so the RI had EMC build a flat-fronted locomotive based on an E-seriesE6B (B unit) but with an operating cab, headlight, pilot, and other features to enable it to operate as an independent locomotive.
EMC AB6
Rock Island No. 751 at Joliet, Illinois in October 1966
Since the small three- and four-car trains the units would have to haul independently were very light, the AB6 pair were built with only one 1,000 hp EMC 567 V12 engine, and a baggage compartment where the second engine would have been. Later, with increasing trainloads, the baggage compartment was replaced with a second engine.
In 1965, the units had their steam generators replaced with head-end power and were reassigned to push-pull suburban service in the Chicago area. In this form, they lasted until the mid-1970s and were eventually scrapped.
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