A Doppler LIght Detection And Ranging (LIDAR or lidar) system is studied for the application of measuring train ground speed in a non-contacting manner, as an alternative to the current train speed measurement devices such as wheel-mounted tachometers or encoders. The ability to accurately measure train speed and distance is a critical part of monitoring track geometry conditions.

Wheel-mounted tachometer speed measurements often fluctuate due to wheel vibrations, change in wheel diameter, or wheel slip affecting the measurement accuracy.  Frequent calibrations are needed to account for changes in wheel diameter due to wear.  Additionally, the high levels of vibrations at the wheel can cause occasional mechanical failure of the encoder.

This thesis examines LIDAR as a non-contact train speed measurement device as a direct retrofit for wheel-mounted encoders. LIDAR uses Doppler technology to accurately measure train speed. The LIDAR system consists of two laser sensors and can be installed on either the car body or the truck on the underside of the train. The sensors measure the true ground speed of each rail, from which the track curvature can then be assessed based on the difference between the right and left rail speeds. The LIDAR train speed, distance, and curvature results are then evaluated against encoder readings and other conventional train measurement devices.

Various tests were performed, including field-testing onboard a track geometry railcar operated by Norfolk Southern for evaluating the efficacy, accuracy, and durability of the LIDAR system; and laboratory tests on a 40-foot rail panel for assessing the ability to obtain measurements at super low speeds.

The test results indicate that when compared with other conventional means used by the railroad industry, LIDAR is capable of accurately measuring train speed and distance from speeds as slow as 0.3 mph and up to 100 mph.  Additionally, the curvature  measurements proved to be as accurate as Inertial Measurement Units (IMUs) that are commonly used in track geometry measurement railcars.