The A and C subway lines in New York City mostly use the 14th Street Tunnel connected to the IND Eighth Avenue Line to travel into and out of Manhattan.
The question pertains to the train lines A and C that operate in New York City and specifically asks which tunnel they use to enter and exit Manhattan. While the reference material provided discusses historical aspects such as the advent of trolleys, the Public Gardens Portal in Boston, and the Underground Railroad, it does not directly mention the current train lines in Manhattan, New York.
Nevertheless, for the purpose of providing a direct answer, the A and C lines primarily utilize the IND Eighth Avenue Line, which runs through the 14th Street Tunnel when traveling to and from Manhattan.
Final answer in 2 lines: The A and C subway lines typically enter and exit Manhattan using the 14th Street Tunnel, part of the IND Eighth Avenue Line.