EMD's answer was a new locomotive based on their existing FP9, but lengthened to accommodate additional equipment, such as a larger train heating steam boiler and third rail contact shoes.

The first 30 locomotives also had a small DC pantograph for use within New York City's Grand Central Terminal, where long gaps exist in the third rail because of the complex trackage that includes numerous railroad switches.

[citation needed] Two batches of FL9s were built; an initial 30 (including the original test units 2000 and 2001, originally built with a Blomberg front truck, but later upgraded following testing) from October 1956 through November 1957 with the older 1,750 hp (1,305 kW) V16 EMD 567C engine; and an additional 30 between June and November 1960 with the newer 1,800 hp (1,342 kW) V16 EMD 567D1 engine.

Early plans to extend the catenary to Boston were never completed due to the perennial financial problems that plagued the New Haven almost continuously from the 1920s to its demise in 1969.

The FL9s allowed through passenger trains from Grand Central Terminal to reach Boston, Springfield, and other non-electrified destinations without the need for an engine change at New Haven.

They were purchased with the intent of allowing the eventual elimination of all New Haven electric locomotives and the abandonment of the electrification east of Stamford, Connecticut, 33 miles from Grand Central.

The fact that the entire New York to Boston line is now electrified shows the short-sightedness of this concept, which had been adopted by the McGinnis management to avoid the cost of modernizing the New Haven's Cos Cob, Connecticut power plant.

Meeting the weight limits of the Park Avenue Viaduct in Manhattan, the FL9 made it possible to eliminate the engine change and allow trains to reach Grand Central in less time.

FL9s were used on the New Haven's premier name train, the Merchants Limited, which covered the 229.5 miles between Grand Central Terminal and South Station, Boston in 4 hours 15 minutes.

Their ability to avoid the engine change in New Haven allowed them to remain in service on trains that travelled in non-electrified territory, and they could also be operated like conventional diesel locomotives.

When the New York Metropolitan Transportation Authority (MTA) began funding these commuter services in 1970, many were repainted blue with a bright yellow nose, although they remained Penn Central-owned.

Still painted in New Haven livery, the unit was depicted pulling a commuter train past the entrance to Lex Luthor's hideout during the villain's introduction scene.