The model S-12 was the sucessor to the DS-4-4-1000. Production ran from mid-1950 to 1956. By 1955, B-L-H began the phase-out of diesel-electric production. During this period the supply of Westinghouse electrical gear for the switcher line became depleted, forcing Eddystone to begin using General Electric main generators and traction motors. Between the time in 1953 that Westinghouse began contemplating its discontinuance of railroad electrical equipment and the time B-L-H decided to go out of the diesel-electric business, a new, 6-inch longer frame was designed to accommodate the GE gear. The frame was also modified so that the end steps and pilots could be built separately and bolted on rather than welded. By 1956 the end was at hand and with the departure of Erie Mining Co. S-12 403 (October, 1956) after its use in testing the New Haven RP-210s, the book was closed. With the erecting halls shut down, the final tally of the S-series switchers could be made. There had been 63 S-8 switchers sold to 16 customers in a production run of three years, nine months. In six years and one month of S-12 deliveries, 452 units had been delivered to 48 original customers.

"As for the old F-M 1000 numbered series, not much to say. All the yards here in Milwaukee used 'em as switchers in the mid to late 60's. Stinky little bastards that also blew carbon sparks out the stacks, but they sure did their share of switching. I often got a chance to run one, the controls were really simple. The throttle was mounted in a sideways U shape. Pull down and back and you went forward, up and back and you went backwards. We used to switch 25 to 35 car drags at a time and those little rascals could kick 'em good."

General Electric built this locomotive (c/n 41104) in October 1978 along with what became C&NW #1199 (c/n 41105) as demonstrators. It weighed 136 tons and was powered by two Cummins six cylinder KTA-1150-L diesels producing 550 horsepower each, totalling 1100 horsepower. The two demonstrators tested for two years on the C&NW before going to the BN and renumbered as 1100 and 1101.