This paper presents an experimental study of a new assessment of track irregularity based on a new inverse analysis method. Two tri-axial accelerometers are mounted on the axel box of a track recording vehicle to measure the dynamic response. And according to the acceleration responses at axel box, the spatial path of the moving wheel on the track will be found by an inverse analysis method. The rotation gradients in three moving coordinate direction along the tracks scanned over by the accelerometers can be clearly identified. These rotation gradients can be used to identify the locations of the geometrical irregularities of the tracks due to settlement, deformation, lose of fastener, crack, and welding point of the inspected track. Some examples are demonstrated to verify the feasibility and accuracy of this innovative theory.