The Princeton Branch is a commuter rail line and service owned and operated by New Jersey Transit (NJT) in the U.S. state of New Jersey. The line is a short branch of the Northeast Corridor Line, running from Princeton Junction northwest to Princeton with no intermediate stops. Also known as the Dinky, or the Princeton Junction and Back (PJ&B), the branch is served by special shuttle trains. Now running 2.7 mi (4.3 km) along a single track, it is the shortest scheduled commuter rail line in the United States. The run takes approximately 5 minutes in each direction.

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. The service is the successor to Pennsylvania Railroad commuter trains between Trenton and New York, and is NJ Transit's busiest commuter rail service. 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. Most servicing is done at the Morrisville Yard, at the west end of the line.