New York Tunnel Extension
The New York Tunnel Extension (also New York Improvement and Tunnel Extension), owned by the Pennsylvania Tunnel and Terminal Railroad, was an important part of the Pennsylvania Railroad system, comprising the tunnels and approaches from New Jersey and Long Island to New York Penn Station in Midtown Manhattan.
Before the tunnel opened in 1910, PRR passengers had to transfer to the Hudson and Manhattan Railroad or a ferry to cross the Hudson River, and LIRR passengers had to take a subway line or ferry across the East River.
The company was formed on June 26, 1907 as a consolidation of the Pennsylvania, New Jersey and New York Railroad and the Pennsylvania, New York and Long Island Railroad, which were the New Jersey and New York parts, respectively. The PNJ&NY was incorporated February 13, 1902, and the PNY&LI was incorporated April 21, 1902.
The PRR merged into Penn Central Transportation in 1968, and all the property of the PT&T was conveyed to Amtrak on April 1, 1976 when Conrail's system was formed. It is now part of Amtrak's Northeast Corridor; New Jersey Transit and the Long Island Rail Road use their respective halves to reach New York Penn Station.
The North River Tunnels pass under the Hudson River and the East River Tunnels pass under the East River. The Long Island side was used mainly by the Long Island Rail Road, then a PRR subsidiary. Many other railroads had trackage rights over one half or the other.
On the New Jersey side, the railroad merged with the main line of the United New Jersey Railroad and Canal Company, the PRR's main line through New Jersey, at Kearny Junction. On the Long Island side, the railroad ended at Sunnyside Yard, with connections to the Long Island Rail Road and New York Connecting Railroad.
Originally, an engine change was made at Manhattan Transfer, since the line wasn't electrified all the way to Philadelphia, but New York City would only allow electric trains to operate through the tunnels.
One branch of the line existed, the Harrison Branch, splitting west off the line just east of its west end, and running west to a connection with the Delaware, Lackawanna and Western Railroad's Harrison Cut-off and the Erie Railroad's Paterson and Newark Branch. This branch was used only for freight.
The following non-PRR railroads used the line:
- Baltimore and Ohio Railroad during World War I 
- Lehigh Valley Railroad
- New York, New Haven and Hartford Railroad
- Manhattan Transfer (PRR station)
- New York Penn Station
- New York Connecting Railroad
- East River Tunnels
- North River Tunnels
- Cross-Harbor Rail Tunnel
- ICC Valuation Report
- Transactions of the American Society of Civil Engineers, Vol. LXVIII, Sept. 1910: The New York Tunnel Extension of the Pennsylvania Railroad (Project Gutenberg eText)