Originally constructed as the Third Avenue Railroad with in Manhattan, that company subsequently expanded into the Bronx and Westchester County by acquiring street railways in those areas.
The Third Avenue Railway Co. was incorporated in 1910, acquiring the Third Avenue Railroad. The following street railway companies evolved into the Third Avenue Railroad within the Bronx.
Third Avenue Railroad - chartered 1853, opened 1858, leased 1898 to 1907 to Metropolitan Street Railway, acquired 1910 by Third Avenue Railway Co.
Bronx Traction Co. - incorporated 1904, consolidating Suburban Traction Co., Van Nest West Farms & Westchester Traction Co., Wakefield & Westchester Traction Co., West Farms & Westchester Traction Co., Williamsburg & Westchester Traction Co., acquired 1904 by Third Avenue Railroad.
City Island Railroad - chartered 1884, opened 1887, merged 1913 into Pelham Park & City Island Railway, acquired 1914 by Third Avenue Railway Co., operations suspended 1919.
Kingsbridge Railway - chartered 1898, acquired 1901 by Third Avenue Railroad.
New York City Interborough Railway - chartered 1902, opened 1906, acquired 1911 by Third Avenue Railway Co.
Pelham Park Railroad - chartered 1884, opened 1887, merged 1913 into Pelham Park & City Island Railway, acquired 1914 by Third Avenue Railway Co., operations suspended 1919.
Union Railway Co. of New York - incorporated 1892, consolidating Harlem Bridge Morrisiana & Fordham Railway Co. and three other companies which had not yet constructed any lines, leased 1898 by Third Avenue Railroad.
Southern Boulevard Railroad - chartered 1885, opened 1895, acquired 1901 by Union Railway Co. of New York.
In 1924, the Third Avenue Railway Co. formed a subsidiary, Surface Transportation Corp., to operate the bus routes which would replace the streetcar routes. And in 1948, the last streetcars were replaced with buses.