Abstract: An apparatus or system for a track-based detection of a wheel profile of passing train wheels, with the apparatus including an illumination unit with a light source and optical elements which transmit light towards a profile of the train wheels. An illumination area of the wheel profile is scanned with a scanning unit and a position of the wheel relative to the rail is scanned with a measuring unit. A data processing unit processes the data from the scanning unit relating to a condition of the wheel and a further unit is provided for transmitting the data concerning the condition of the wheels to at least a central processing unit.