With the increase of passenger traffic on the New Haven's Shore Line requiring trains of fourteen to sixteen cars, the capacity of the I4 Pacifics which had been used for twenty years was severely taxed. As a result of tests with two of these engines with a twelve-car train operating over the 156.8 miles between New Haven and Boston, this new design was worked out. Much research went into these studies, which included consideration of the 0.6 to 0.7 per cent ruling grade near Sharon Heights outside of Boston, and the eventual plans called for a locomotive which could maintain a 60-mile speed over these grades with a twelve-car 830-ton train.
Ten of these Class I5 engines were delivered in 1937, being partly streamlined but with maintenance accessibility in mind. They have been satisfactory in every respect since they went into service and will probably remain the last steam locomotives to be purchased by the New Haven.
The first of the class, #1400, was built by the Baldwin Locomotive Works in 1937. The last ones were withdrawn in 1951 with a life of only 14 years - a considerable waste of resources. Had they survived a few years longer, they would have undoubtedly became ideal candidates for preservation.