In the fifties the New Haven had the problem that the last stretch of track into New York's Grand Central Terminal had to be operated by electric locomotives, but the lines outside of the city were not electrified. So they had to switch between electric and diesel locomotives and needed an electro-diesel locomotive that could operate on the whole line. The solution was the EMD FL9 that was based on the FP9.

It had been made longer to provide space for additional equipment and had contact shoes to pick up direct current from the third rail. The rear truck received an additional non-powered axle to keep the axle load low. The first batch of 30 locomotives was powered by an EMD 16-567C with 1,750 hp. Another batch of 30 was built in 1960, these got the 16-567D1 with 1,800 hp. The New Haven designated these EDER-5 and EDER-5A.

After the New Haven had removed the overhead lines from many lines, these locomotives could replace old electric locomotives. Occasionally, they also had to haul freight trains. They later became part of Penn Central, then Conrail and finally Metro-North. Additionally, twelve were sold to Amtrak and others to the Connecticut Department of Transportation (CTDOT). Most were retired in the nineties, but the last ones remained in service until 2009.