When the New York, New Haven & Hartford received its first ten 4-8-2s (road numbers 3300 through 3309) they were "Light Mountains" assigned by the USRA and delivered by the American Locomotive Company in 1919. These Class R-1 "mountains" performed very well, pulling up to 20 car passenger trains with few problems. The NYNH&H ordered thirty more 4-8-2s (road numbers 3310 through 3339 and designated Class R-1a) from ALCO and took delivery of them in 1920. Nine more (road numbers 3340 through 3348 and designated Class R-1b) arrived from ALCO in 1924. These 49 locomotives had 27 x 30 cylinders, 69" drivers, a 200 psi boiler pressure and exerted 53,900 lbs of tractive effort. Each weighed about 334,000 pounds.

Also, in 1924, a single heavier 4-8-2 was delivered by ALCO. This first of a new class had 27 x 30 cylinders, 69" drivers, a 250 psi boiler pressure a resultant tractive effort of 63,390 pounds. This locomotive Classified R-2 was nicknamed "The Mayflower" and was put to work in passenger service. In 1926, seven more (road numbers 3501 through 3507) Class R-2a "Mountains" were added to the roster. These last seven had 27 x 30 cylinders, 69" drivers, a very high boiler pressure of 265 psi and a tractive effort of 71,400 lbs. Later, the cylinders were reduced to 25 inches which changed the tractive effort to 61,200 pounds.

Three 4-8-2s were delivered by ALCO in 1926 and were given road numbers 3550, 3551 and 3552. They bore a similarity to the Class R2a locomotives, but had three cylinders and were designated Class R-3. Another ten (road numbers 3553 through 3562) three cylinder locomotives (Class R-3a) arrived in 1928. Each of the 13 "Mountains" had a total weight of 374,000 lbs, (3) 22 x 30 cylinders, 69" drivers, a 265 psi boiler pressure, and exerted 71,000 lbs of tractive effort.

All seventy of the NYNH&H Mountains were retired by 1951 and were scrapped shortly after.