The TRTA Hibiya Line -- now operated as the Tokyo Metro Hibiya Line -- runs north to south through the heart of Tokyo, connecting Kita-Senju in the northeast to Naka-Meguro in the southwest, where it links with the Tobu Railway's network. At roughly 9 a.m. on that Wednesday morning, an eight-car southbound train was approaching Naka-Meguro Station when its last car jumped the rails on the sharp curve immediately before the platform. The derailed car swung outward, fouling the adjacent track. Moments later, the fifth and sixth cars of a northbound eight-car Tobu Railway train, heading from Naka-Meguro toward Takenotsuka, slammed into the stricken car. The impact was violent enough to crumple carriages and trap passengers inside the wreckage.

The physics of the accident were almost inevitable once the derailment began. On double-track sections where the distance between parallel rails leaves only about 600 millimeters of clearance between passing trains, any car that leaves its track on the side facing the opposing line will almost certainly obstruct it. A collision becomes not a possibility but a certainty. Making matters worse, the track circuit safety system -- which detects trains by sending electrical current through the rails and registering when an axle shorts the circuit -- could not sense the intrusion. The derailment hadn't broken the opposing track's rails, so no fault signal was generated. The oncoming Tobu train received no automated warning. In such cases, the only defense is human: a crew member must physically warn signal staff, wave a red flag, or manually short the adjacent track circuit using a specialized clip. In the seconds between derailment and collision, there was no time for any of these measures.

Five passengers died in the wreckage, and 63 more were injured. Emergency crews worked to extract survivors from the mangled carriages in the tight space of the underground approach to Naka-Meguro Station. The disaster prompted an investigation into curve speeds, track geometry, and the limitations of track circuit safety systems on sections where trains pass in close proximity. It raised uncomfortable questions about infrastructure that had been designed for an earlier era of lighter, slower rolling stock and whether it had kept pace with the demands of one of the world's most heavily used rail networks. Tokyo's subway system carries millions of passengers daily, and the margin for error is razor-thin.

In the years since the disaster, the site near Naka-Meguro Station has become a place of quiet remembrance. Tokyo Metro has observed the anniversary of the accident, and the 25th anniversary in March 2025 drew renewed attention to both the victims and the safety improvements that followed. The accident shares a grim kinship with similar disasters worldwide -- the 1995 Ais Gill accident in the United Kingdom, where a landslide derailed a train across both tracks into the path of an oncoming service, and the 1993 Big Bayou Canot disaster in Alabama, where misaligned rails that didn't physically separate went undetected by track circuits. Each case exposed the same vulnerability: automated systems that assume intact rails cannot protect against threats they were never designed to detect. The lesson of Naka-Meguro is written in the geometry of that curve and the silence of the circuits that should have screamed.