M line

M line may refer to:

• The McKinney Avenue Transit Authority streetcar line, in Dallas, Texas, U.S. – also designated as the M-Line
• Ashmont–Mattapan High Speed Line, Boston, Massachusetts – streetcar line known as the M Line
• The M Ocean View line in San Francisco – a light rail line and former streetcar line
• M Train (disambiguation), one of several train services
• HP M-series, a line of digital cameras
• M line (mittel line or middle line), a structure in muscle; see Myofibril or Sarcomere
• Geometrisches Mittel, or Geometric mean