S Line may refer to:
Rail transportation S-Bahn, a type of hybrid urban and suburban railway. S-train (Copenhagen), an urban rail network in Copenhagen, Denmark Milan S Lines, a commuter rail network in Milan, Italy Main Line (Seaboard Air Line Railroad) S-Line Raleigh to Richmond project, a project to restore service to an abandoned portion of the Seaboard Air Line Railroad main line S-Line (Norfolk Southern), a railroad line running between North Carolina and Tennessee operated by the Norfolk Southern Railway in the United States Sounder commuter rail#S Line, a commuter rail line serving Seattle, Washington, United States S Castro Shuttle, a Muni Metro line in San Francisco, California, United States S (New York City Subway service), a subway shuttle service in New York City, New York, United States S Line (Utah Transit Authority), a streetcar connecting the cities of Salt Lake City and South Salt Lake that is operated by the Utah Transit Authority in Utah, United States S (Los Angeles Railway), a former streetcar service in Los Angeles, California
Other uses Audi S line, an optional sports trim packages available on various Audi cars S Line (ice hockey), a forward line for the Montreal Maroons, a former NHL hockey team S Line (TV series), a South Korean 2025 TV series Nikon S-line lenses, Nikon branding for especially high-performance ("superior") Z-mount lenses