The New York, New Haven and Hartford Railroad was formed on July 24, 1872, through the consolidation of the New York and New Haven Railroad and Hartford and New Haven Railroad. It owned a main line from New York City to Springfield, Massachusetts via New Haven and Hartford, Connecticut, and leased other lines, including the Shore Line Railway to New London. The company later leased more lines and systems, eventually forming a virtual monopoly in New England south of the Boston and Albany Railroad.
The first line of the original system to open was the Hartford and New Haven Railroad, opened from Hartford to New Haven, with steamship connections to New York in 1839, and to Springfield, with rail connections to Worcester and Boston, in 1844. The New York and New Haven was built later, as it ran parallel to the Long Island Sound coast and required many bridges over rivers. It opened in 1848, using trackage rights over the New York and Harlem Railroad (later part of the New York Central Railroad system) from Woodlawn in the Bronx south to Manhattan. With the opening of Grand Central Terminal in 1913, New Haven's New York City terminal was moved there.