The Atlantic Branch is an electrified rail line owned and operated by the Long Island Rail Road in the U.S. state of New York. It is the only LIRR line with revenue passenger service in the borough of Brooklyn.

The South Side Railroad of Long Island was a railroad company in the U.S. state of New York. Chartered in 1860 and first opened in 1867 as a competitor to the Long Island Rail Road, it was reorganized in 1874 as the Southern Railroad of Long Island and leased in 1876 to the LIRR. After a reorganization as the Brooklyn and Montauk Railroad in 1879 it was merged in 1889.