A railway positioning system provides an on-board speed measurement device (6) inducing eddy currents in the wayside structure at two spots along the travelling direction, measuring the variations of the magnetic field emitted by the wayside structure and determining position and speed by correlating the 2 measured signals known from U.S. Pat. No. 5,825,177 and a wayside coded tag (1) providing a coding recognizable by the onboard speed measurement device (6). The coded tag (1) consists of a bar (4) with several slots (3) in which metal blocks (2) of different sizes are mounted. The block sizes and positions are selected to represent a coding according to Quadrature Amplitude Modulation.