In the mid-1930s, the New Haven had a requirement to haul a twelve-car (830 short tons) train at 60 mph (97 km/h) on a 0.6 to 0.7 percent grade. Since only 40 or 45 mph were allowed in many places in the New Haven network, good acceleration was also necessary. Due to the axle load being limited at the same time, a locomotive with a 4-6-4 wheel arrangement was developed for these requirements.
collection Taylor Rush
The ten Class I-5 locomotives were the last new steam locomotives of the New Haven. They were the first in the US to have partial streamlined fairing that still allowed good accessibility for maintenance. The last I-5 class locomotive was retired in 1951.