The existing railway lines need to be updated with the ageing of railways and the demand for capacity increase,thus the surveying of railway lines is necessary without affecting its routine operations.However,the surveying of most of the existing railway lines still relies on traditional manual methods,which are characterized by low efficiency,heavy task,complex procedures,and repeated online.To address these priblems,this paper uses a least squares based 2D template point cloud matching approach to jointly and repeatedly calculate the rail track surface centre point position and then obtain the track centre line,track gauge and other related parameters based on absolute 3D point cloud data obtained by the track mobile laser scanning system.The test and result analysis at the track test site show that the proposed approach can effectively extract the track centre line and provide accurate centre line data for the existing line survey.