In 2015, a truly lightweight track geometry measuring trolley was successfully developed to enable real-time measurement after all track maintenance work. A commercial product model named the LR-S100 was realized that is small (disassembled total length of 1,600 mm maximum) and lightweight (13.4 kg, which is about one-third that of the track geometry measuring trolley the LIGHTREC). Seven items may be measured in real-time: track gauge, cross level, alignment (right and left), longitudinal level (right and left), and twist. The measuring accuracy is high, and it far outperforms current track geometry measuring trolleys in terms of repeatability error. The mechanism of the LR-S100 is very simple and easy to maintain because there are no moving parts for measuring alignment and longitu- dinal level. These advantages, which are not found in any similar conventional product, were achieved with a brand-new track geometry measuring principle called the ‘differential-difference method’ for the LR-S100. Since the measurement accuracy of the LR-S100 is high, it is also possible to restore the actual track geometry and to compensate for errors in gauge and cross-level irregularities by means of a three-wheel trolley without any additional sensors.
actual track geometry, D1, Kalman ﬁlter, MEMS gyroscope, track geometry measuring trolley, versine
 Naganuma, Y., Tanaka, M. & Ichikawa, K., High-speed track inspection car in the new Dr. Yellow. Proceedings of the World Congress on Railway Research (WCRR 2001), 2001.
 Naganuma, Y., Fukushima, S. & Morimoto, M., Truly portable track recording equipment and its application for track quality management. Proceedings of the World Congress on Railway Research (WCRR 2003), pp. 1313–1323, 2003.
 Naganuma, Y. & Yada, T., Development of truly portable track geometry recording trolley and accompanying new measurement principle. Computers in Railways XV: Railway Engineering Design and Operation, 162(14), pp. 329–342, 2016.
 Chen, Q., Niu, X., Zuo, L., Zhang, T., Xiao, F., Liu, Y. & Liu, J., A railway track geometry measuring trolley system based on aided ins. Sensors, 18(2), p. 538, 2018.
 Esveld, C., Modern Railway Track. MRT-Productions, 1989.
 Kiyono, S., Kume, T., Asakawa, Y. & Kamada, O., Differential laser auto-collimation method (2 report). Journal of JSPE, 57(4), p. 652, 1991 (in Japanese).
 Naganuma, Y., Kobayashi, M. & Okumura, T., Inertial measurement processing techniques for track condition monitoring on Shinkansen commercial trains. Journal of Mechanical Systems for Transportation and Logistics, 3(1), pp. 315–325, 2010.
 Yoshimura, A. & Naganuma, Y., Bayesian reconstruction of 3D railway track geometry by particle filter. Proceedings of the Second International Conference on Railway Technology, 2014.
 Naganuma, Y. & Uematsu, T., Reconstruction of actual track geometry using data measured by inertial trolley. WIT Transactions on the Built Environment, WIT Press, 181, 2018.