The 7 train runs through the Steinway Tunnel beneath the East River.
The tunnel connects Manhattan and Queens, carrying the IRT Flushing Line between Grand Central–42nd Street in Manhattan and Vernon Boulevard–Jackson Avenue in Queens. It was built to accommodate the original IRT subway technology, including relatively narrow, low-clearance cars.
The Steinway Tunnel opened for rapid-transit service in 1915. It had originally been constructed for trolley use, but its steep grades made it unsuitable for conventional streetcars. The tunnel was therefore adapted for subway operation.
The 7 and 7 Express share the Flushing Line. The express version is marked by a diamond symbol, while the local service is marked by a circle; both use the Steinway Tunnel.