The first nine New York, New Haven & Hartford "Pacifics" were delivered from the American Locomotive Company's Schenectady Works in September 1907. These locomotives were designated as Class I-1 and were given road numbers 1000 through 1008. They had 22 x 28 cylinders, 73" drivers, a 200 psi boiler pressure, exerted 31,600 lbs of tractive effort and each weighed 229,500 pounds. In September and October of 1907, the Baldwin Locomotive Works delivered 21 more Class I-1 which were very similar to the ALCO locomotives and they were given road numbers 1009 through 1029. A final pair of Class I-1 locomotives (road numbers 1030 and 1031) arrived from ALCO in 1910 and were very similar to the earlier ones. Between 1912 and 1923 all 32 of the Class I-1 locomotives were reworked. The cylinders were enlarged to 23" diameter, superheaters were added and the total weight was increased to 234,400 pounds. The change in the cylinder diameter increased the tractive effort to 34,493 lbs.

In just a few years the length and weight of passenger trains were increasing and the I-1 "Pacifics" were having difficulty keeping up. So the NYNH&H designed and ordered new "Pacific" with more power. Fifty of the new Class I-2 came from ALCO in May and June of 1913. These locomotives (road numbers 1300 through 1349) had 24 x 28 cylinders, a larger boiler, but the same 73" diameter drivers as the I-1s. These new Class I-2 "Pacifics" could exert 37,558 lbs of tractive effort.

There were six other new "Pacifics" added to the roster in 1913. They were built by the Baldwin Locomotive Works, designated as Class I-3 and assigned road numbers 1090 through 1095. These locomotives had 24 x 28 cylinders, 79" drivers, a 200 psi boiler pressure, exerted 34,705 lbs of tractive effort and each weighed 246,000 pounds.

The last group of NYNH&H "Pacifics" were the fifty Class I-4 locomotives delivered by ALCO's Schenectady Works in 1916. These locomotives were assigned road numbers 1350 through 1399 and were designed to pull the heaviest passenger trains without double heading. They had 26 x 28 cylinders, 79" drivers, a 200 psi boiler pressure, exerted 40,731 lbs of tractive effort and each weighed 266.000 pounds. The I-4s were the top passenger locomotives on the NYNH&H for more than 20 years from 1916 until the mid 1930s.

Most of the NYNH&H "Pacifics" were used continuously through World War II, but were all retiired by 1952. There are no surviving examples of these locomotives.