See this tutorial: Cartographic Symbol Construction with MapServer.

IMAGETYPE agg_png

SYMBOL
  NAME "circle"
  TYPE ELLIPSE
  FILLED TRUE
  POINTS
    1 1
  END
END

SYMBOL
  NAME "rectangle-railway"
  TYPE ELLIPSE
  POINTS
    1 1
  END
  FILLED TRUE
  STYLE
    8 12 8 12
  END
END

LAYER
  NAME "Railways"
  STATUS DEFAULT
  TYPE LINE
  DATA "railways.shp"
  CLASS
    MAXSCALEDENOM 500000
    STYLE
      COLOR 102 102 102
      SYMBOL "circle"
      SIZE 4
    END
    STYLE
      COLOR 255 255 255
      SYMBOL "rectangle-railway"
      SIZE 2
    END
  END
  CLASS
    MAXSCALEDENOM 10000000
    STYLE
      COLOR 102 102 102
    END
  END
END

See also this page about vector symbols which follow the line orientation (negative GAP).

SYMBOL
  NAME 'triangle'
  TYPE VECTOR
  POINTS
    0 0
    0 1
    1 0
    0 0
  END
  FILLED TRUE
  GAP 2
END

LAYER
  NAME "curves"
  STATUS DEFAULT
  TYPE LINE
  DATA "curves.shp"
  CLASS
    STYLE
      COLOR 128 16 16
    END
    STYLE
      COLOR 212 32 32
      SYMBOL 'triangle'
      ANGLE 35
      SIZE 12
    END
  END
END

SYMBOL
  NAME "circle"
  TYPE ELLIPSE
  POINTS
    1 1
  END
  FILLED true
END

LAYER
  NAME "curves"
  STATUS DEFAULT
  TYPE LINE
  DATA "curves.shp"
  CLASS
    STYLE
      SIZE 1
      COLOR 0 0 0
    END
    STYLE
      SYMBOL "circle"
      SIZE 6
      COLOR 212 32 32
      OFFSET 1 -3
    END
    STYLE
      SYMBOL "circle"
      SIZE 6
      COLOR 32 212 32
      OFFSET 1 3
    END
  END
END