The HHe9y Class is a class of Transfer locomotives designed by Reginald Thornton at the Eastern & Atlantic's Trenton works.
The Eastern & Atlantic had 5 major yards in the New York City area, in Newark, Union City, Jamaica, Flushing and Yonkers. This required a large amount of transfer locomotives to transfer trains between each yard. In the early 1920's, transfer locomotives were typically 2-8-0's and some larger tank engines. However, with the large upturn of traffic in the mid 1920's, the E&A found that these locomotives were becoming ineffective. Thus, a much larger locomotive, an articulated, was needed.
Reginald Thornton had been hired in 1921, and his first articulated locomotive was the HHfmt9 Class. The locomotives were not meant for high speeds, but rather for high pulling power at low speeds.
The HHe9y Class was extremely successful, especially around the tight curves of New York City. 15 of this class were built in the Trenton works in 1924-1926, and a further 26 of them were sent to similar large cities, like Washington, Atlanta and Miami. A modified version of the design was used in marshalling yards, the HHs10y Class.