James A. Farley Post Office Building, 1912.

Formerly the General Post Office Building, the main branch of the United State Postal Service was built as part of the Pennsylvania Station construction plan.

The firm of McKim, Mead, and White designed both the train station and the post office in a Beaux-Arts style. The General Post Office and Pennsylvania Station became twin symbols of American progress and industry.

With its giant order Corinthian colonnade stretching the length of two city blocks, the Post Office matched the grandiosity of Penn Station across the avenue. Bracing either side of the colonnade are square pavilions, capped with low domes expressed as stepped pyramids on the exterior. From the street, continuous steps lead to the main floor, which hosted various customer services. A dry moat at the base of the building allows for natural light to flood into the basement level, occupied by workspaces. On the interior, the ceiling is coffered and its floor is Knoxville marble.

The colossal structure housed nearly one-acre of mail sorting space. In 1967, the facility was praised for handling one-tenth of all U.S. mail during Christmastime. However, the facility became outdated, hand-sorting technology just didn't cut it any longer, and the size of the building became too much to maintain.

The former post office hosted a variety of bizarre functionalities: Operation Santa, beginning in 1912, authorized postmasters to respond to children's letters to Santa Claus. Pneumatic mail tubes facilitated the transport of mail (up to 100 pounds) to 22 post offices in Manhattan (and one in Brooklyn) at speeds of 30 to 35 mph. A Museum of Postal History occupied a small portion of the main floor, displaying postal paraphernalia until 2017.

Portions of the landmark are now being converted to house the Moynihan Station, an Amtrak concourse which would expand Penn Station.