In May 1859, the New Jersey Railroad began work on the project. The completion of the second track was authorized by the Board on April 23, 1859. The New Jersey Railroad Board approved a contract for the rails for the second track between New Brunswick and Rahway to Cooper & Hewitt on February 5, 1859. In December 1855, work to extend the line's second track from Elizabethtown to Rahway was completed, eliminating the need for trains to meet in single-track territory. Service on what is now the Northeast Corridor dates to the 1830s, with trains originating and terminating at the PRR's terminal at Exchange Place in Jersey City, which was the terminus of the PRR's network for most of the 19th century. Connecting SEPTA Trenton Line service between Philadelphia and Trenton is listed in the timetable. The Princeton Branch is a shuttle service connecting to the line. The Northeast Corridor Line is colored red on NJ Transit system maps and its symbol is the State House. Most servicing is done at the Morrisville Yard, at the west end of the line. After arrival at New York Penn Station, some trains load passengers and return to New Jersey, while others continue east to Sunnyside Yard for storage. The service is the successor to Pennsylvania Railroad commuter trains between Trenton and New York, and is NJ Transit's busiest commuter rail service.

The Northeast Corridor Line is a commuter rail service operated by NJ Transit between the Trenton Transit Center and New York Penn Station on Amtrak's Northeast Corridor in the United States.

Njtransit trains