August Belmont Jr. was an American financier. He financed the construction of the original New York City Subway line (1900–1904) and for many years headed the Interborough Rapid Transit Company, which ran the transit system. He also financed and led the construction of the Cape Cod Canal in Massachusetts, which opened in 1914. Belmont bought the land for and built New York's Belmont Park racetrack—named for his father—and was a major owner/breeder of thoroughbred racehorses. He served as chairman of the board of the Louisville and Nashville Railroad. He also served as a director of the Southern Pacific Co., parent of the railroad, and National Park Bank.

The first regularly operated line of the New York City Subway was opened on October 27, 1904, and was operated by the Interborough Rapid Transit Company (IRT). The early IRT system consisted of a single trunk line running south from 96th Street in Manhattan, with a southern branch to Brooklyn. North of 96th Street, the line had three northern branches in Upper Manhattan and the Bronx. The system had four tracks between Brooklyn Bridge–City Hall and 96th Street, allowing for local and express service. The original line and early extensions consisted of:The IRT Eastern Parkway Line from Atlantic Avenue–Barclays Center to Borough Hall The IRT Lexington Avenue Line from Borough Hall to Grand Central–42nd Street The IRT 42nd Street Shuttle from Grand Central–42nd Street to Times Square The IRT Broadway–Seventh Avenue Line from Times Square to Van Cortlandt Park–242nd Street The IRT Lenox Avenue Line from 96th Street to 145th Street The IRT White Plains Road Line from 142nd Street Junction to 180th Street–Bronx Park