The Center City Commuter Connection (CCCC), commonly referred to as "the commuter tunnel", is a passenger railroad tunnel in Center City Philadelphia, Pennsylvania, The tunnel was built to connect the stub ends of the two separate regional commuter rail systems, which were originally operated by Pennsylvania Railroad and the Reading Company, two rival rail companies.

Center City includes the central business district and central neighborhoods of Philadelphia, Pennsylvania, United States. It comprises the area that made up the City of Philadelphia prior to the Act of Consolidation, 1854, which extended the city borders to be coterminous with Philadelphia County.