Point Cloud-Based Measurement of Switch Rail-to-Sleeper Longitudinal Displacement with Multi-Bolt Reference

The longitudinal displacement of the switch rail relative to the sleeper is a critical defect affecting the safety of railway switching operations, as the sleeper serves as the mounting base for the switch machine. Existing measurement methods that rely on the stock rail as a reference are susceptible to reference drift and cannot directly capture the displacement of the switch rail relative to its actuation base. This paper proposes a measurement method for the longitudinal displacement of the switch rail relative to the sleeper using the centers of multiple anchor bolts as reference benchmarks. Based on the fact that both the anchor bolts and the switch machine are fixed to the sleeper, the displacement of the switch rail relative to the bolt centers is equivalent to its displacement relative to the switch machine. Multiple anchor bolt centers on the sleeper are used to establish a stable reference frame. A self-developed mobile measurement device is employed to acquire three-dimensional structured light point clouds of the switch rail area. The coordinates of multiple anchor bolt centers are extracted through hexagon fitting. Abnormal coordinates are eliminated via joint adjustment of the multi-bolt centers, establishing a stable spatial reference frame that enables precise measurement of the longitudinal displacement of the switch rail relative to the sleeper. Field experiments on an operational No. 18 turnout successfully extracted the switch rail tip position and a longitudinal misalignment of −2 mm between the two switch rails. In simulation experiments, a creep displacement of 10 mm and multiple gross errors were artificially introduced; after three iterations, the final measured creep was 9.65 mm, with a deviation of only 0.35 mm, verifying the method’s accuracy and robustness under adverse conditions. Additional experiments on a No. 12 turnout with straight and curved switching states demonstrated that the standard deviation of the longitudinal spacing measurements between bolt centers was 1.17 mm under track switching conditions, confirming the method’s stability in the presence of switch machine movement. The proposed method achieves non-contact, automated measurement of the switch-rail-to-sleeper relative displacement, providing a practical technical pathway for turnout condition monitoring.