The New Haven only operated two classes of Mikados, all of which were built by ALCO-Schenectady in 1916. All had drivers of 63 inches and a grate area of 59.2 square feet, but the two classes differed in size. The largest part was made up of the lighter J-1 which were numbered 3000 to 3024. They had cylinders of 25 by 30 inches which were controlled by outside Baker valve gear.

Without their tender, the J-1 weighed 126 tons. They had a combustion chamber with a surface of 59 square feet. Of three thermic syphons with a total surface of 69 square feet, two were mounted in the firebox and one in the combustion chamber. The boiler pressure was originally 180 psi, but was increased to 200 psi in the thirties. The tenders contained 12 tons of coal and 7,500 gallons, but were later replaced by a bigger one with 14 tons of coal. They were retired between 1947 and 1956.

The larger J-2 weighed 155 tons and was intended for steeper grades. They were numbered 3100 to 3107, although the last three had originally been intended for the Central New England Railroad. Their combustion chamber measured 71 square feet, but instead of thermic syphons they had four arch tubes with 28 square feet in total. These locomotives could not convince on steep lines as they did not have mechanical stokers and did not produce enough steam in some situations. So they were quickly transferred to easier grades and ended as helpers. Their end already came in 1946.