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Distributed acoustic sensing (DAS) using existing optical fiber cables facilitates high‐density seismic observation. However, few studies have examined the reliability of the seismic waveform amplitude recorded by DAS. In this study, a DAS network was connected to optical fiber cables installed over a distance of 75 km along a high‐speed train (Shinkansen) railway in the Kumamoto prefecture, Japan. We successfully observed strong motions of the Mj6.6 earthquake (approximately 150 km from the fiber) on 22 January 2022, in Hyuga‐nada, in addition to several small local earthquakes. The observed strong motions from the Mj6.6 earthquake, using DAS, exhibited cycle skipping (clipping) issues due to dynamic range limitations at numerous channels. To address this, we estimated the shaking map, representing maximum strain distributions for Mj6.6, by replacing the clipped data with information from nearby unclipped channels and scaling their RMS amplitudes based on S‐coda (unclipped). Furthermore, we verified the reliability of the amplitude information obtained from DAS by estimating the distance attenuation of seismic waves while correcting for the differences in the structure type and coupling as much as possible. The distance attenuation property of local earthquakes was consistent with that of the peak ground velocities obtained from seismometers, indicating that DAS data acquired using fibers installed on infrastructure (various structures) can also be utilized to assess the spatial distribution of the relative amplitude values along the fiber. Obtaining high‐density seismic motion distributions is important for earthquake early warning and accurate damage estimation of strong motions. Plain Language Summary: In the past decade, the use of distributed acoustic sensing (DAS) for seismological purposes has grown rapidly. By utilizing existing dark fibers, such as telecommunication cables, high‐density seismic observation networks can be established in residential areas. In this study, DAS was applied to an existing optical fiber cable installed along a railway line, which recorded strong ground motions of Mj6.6 originating from a subduction zone, as well as numerous regional earthquakes. Although the technique uses optical phase differences for strain measurements and therefore has a poor response to strong ground motions, we resolve this issue using the correct data from neighboring stations as a reference and estimated the shaking distribution every 5 m along 75 km of the railway line. Additionally, when we analyzed the distance attenuation of small regional earthquakes, we found that the attenuation characteristics were similar to those obtained from permanent seismometers. These findings will significantly impact society in the future, particularly as the fiber cables replace conventional seismic observation networks. As a result, ultra‐high‐density observation networks will be capable of issuing more accurate earthquake early warnings and estimating damage following earthquakes. Key Points: We successfully observed strong motion from the Mj6.6 earthquake in Hyuga‐nada using Distributed acoustic sensing (DAS) over 75 km along railwayThe shaking map of the Mj6.6 was estimated by replacing the clipped channels with nearby unclipped channels and scaling their RMS amplitudes of S‐codaDistance attenuation property of local earthquakes of DAS consistent with that of peak ground velocities from continuous seismometers [ABSTRACT FROM AUTHOR]