# In-Motion, Non-Contact Detection of Ties and Ballasts on Railroad Tracks
## 4 Experiments
### High-Speed Testing  
Although the earlier branch-line track testing indicates that the LiDAR system can successfully differentiate between ties and ballasts, these runs were performed at low speeds, usually less than 4 mph (6.4 km/h). The low speeds were due to the speed limitation of the track cart used for the tests and the need for safe operation while following the cart. Due to lack of access to track time and standard Hyrail vehicles, it was decided to answer the question about how well the system can perform at higher speeds by conducting a series of tests with a road vehicle over a simulated track consisting of alternating wood planks and ballasts. The main concern was the adequacy of the selected sampling rate, specifically, having sufficient data points on the surface of the ties and ballasts to adequately identify them with low numbers of false positives and false negatives. The simulated track on an asphalt surface outside the RTL facility and the system installation on the rear of a Chevy Silverado pickup truck are shown in Figure 7. Figure 7a displays the simulated track that consisted of 13 planks with the same width as an 8-inch tie (\(\sim\)20 cm) and a length of 2 ft (\(\sim\)0.66 m). The planks were painted to resemble the tie  
Figure 6: A sample of track testing results performed on a branch track and a mainline: (**a**) measured distances from LiDAR sensors to the tie/ballast surfaces on a branch track, with the left (green) plot shifted for better visualization of the plots; (**b**) measured distances from LiDAR sensors to the tie/ballast surfaces on a mainline, with the left (green) plot shifted. Like the previous plots, orange-shaded areas are associated with the ties, and the unshaded regions are associated with the ballasts. surface color. The space between the planks was filled with ballast nearly 3 to 4 cm lower than the top surface of the planks to provide a clear distinction between ties and ballasts in the data. Figure 7b shows the left and right LiDAR sensors mounted to an aluminum structure attached to the hitch of a Chevy Silverado pickup truck. This installation proved to be accessible and adequate for the intended tests. This setup was chosen to ensure safe operation while closely replicating real-world conditions during high-speed testing with Hyrail vehicles. A road vehicle, due to its nature, introduces more stochastic vibrations compared to standard Hyrail vehicles, as they have a more flexible suspension system [17; 18], presenting a worst-case scenario. However, after initial testing, it was determined that the system effectively mitigated the impact of these unwanted vibrations, still capturing satisfactory data. As shown in Figure 8, despite the vehicle's vibrations, the sensors successfully recorded the surface figure of the simulated track. Figure 8 shows a segment of the measured distances from the LiDAR sensors to the surface of the ties and ballasts at 19 and 37 mph (30 and 59 km/h). Comparing the measurements with the track's setup indicates that the sensors correctly identified the width of the ties and the gaps between them, which were filled with ballast. Beyond the sample measurements shown in Figure 8, we performed tests at speeds ranging from 4 to 37 mph (6.4 to 59 km/h), specifically at 19, 31, and 34 mph (30, 50, and 55 km/h). Although not included here, for brevity, the results for other speeds are similar to those shown in Figure 8, and they similarly indicate the success of the LiDAR system in sampling the ties and ballast sections. Interestingly, the results in Figure 8 and other measurements indicate that the LiDAR sensors successfully capture irregularities associated with tie and ballast surfaces, with the ballast exhibiting more significant surface variations than the ties, as expected. Comparing Figure 8a,b shows a notable consistency in measurements. The tie surface features in Figure 8b closely resemble those in Figure 8a, proving the system's repeatability.