M line may refer to:
M-line (mittel line or middle line), a structure in a muscle sarcomere M Ocean View, a light rail and former streetcar line in San Francisco, California M-Line Trolley, a streetcar line in Dallas, Texas M (Los Angeles Railway), a former streetcar service M (SEPTA Metro), a light metro in Montgomery County, Pennsylvania. Geometric mean
See also M Train (disambiguation), several train services M1 (disambiguation)#Railways Line M (disambiguation)