The BL1 prototype and its BL2 production version were diesel-electric locomotives which represented the transition between carbody and road switcher locomotives for EMD. After ALCO had already invented the road switcher type and Baldwin and Fairbanks-Morse had followed, EMD still produced their carbody units of the E and F series. Although their design was well liked, rear visibility was poor and switchmen had problems to ride along with the locomotive on its outside. So the BL1 prototype was completed in September 1947 that had still the old bridge-truss body construction, but a switchman's platform at the front.

It was based on the F3 and had the same 1,500 hp engine. A steam generator for passenger trains was offered as an option. Nevertheless, the around view of the BL1 was not optimal compared to real road switchers and the construction of the carbody structure was more expensive than the load bearing frame of other locomotives. So only a total of 58 production BL2 were ordered, with the Chesapeake & Ohio as the biggest customer with 14 locomotives. After only a short production run, it was replaced by the GP7 road switcher.