It began with a freight train, No. 287, bound for Mito. As the train approached Mikawashima Station to merge onto the main Joban Line, it ran a red signal. A fail-safe mechanism activated, diverting the freight train onto a safety siding to prevent a head-on collision with oncoming traffic. But the massive JNR Class D51 steam locomotive was traveling too fast to stop within the siding. It derailed, and the locomotive and its leading tanker wagon came to rest fouling the main line, their bulk jutting across the tracks like a barricade. At that same moment, 9:36 PM, a seven-carriage passenger train designated 2117H was pulling out of Mikawashima Station heading in the opposite direction. It struck the wreckage of the derailed freight. The collision was not catastrophic in itself; 25 passengers were injured, but the train remained largely intact.

What happened next was a failure not of machinery but of communication. The signal box operators near Mikawashima were overwhelmed by the first collision. In the confusion of coordinating emergency response for the freight derailment and the damaged passenger train, they failed to notify other traffic on the line that the tracks were blocked. No stop signals were sent. No warnings were issued. The passengers of train 2117H, following standard emergency procedure, had opened the escape handles and were walking along the tracks in the dark, making their way back toward the station lights. For six minutes, the main line remained open and unprotected, with dozens of people walking between the rails.

At approximately 9:42 PM, inbound passenger train 2000H rounded the curve toward Mikawashima at full line speed. The driver had received no warning. The train plowed into the wreckage of the derailed 2117H and into the passengers walking along the tracks. The first carriage of 2000H was destroyed on impact. The next three derailed. People who had survived the first collision with only minor injuries were struck and killed on the open track. The final toll was staggering: 160 dead and 296 injured, making it the third-deadliest rail disaster in Japanese history, surpassed only by the 1963 Tsurumi rail accident and the 1947 Hachiko Line derailment.

Before Mikawashima, Japanese National Railways operated under a doctrine that prioritized keeping trains moving. The standing policy was "do not stop trains unless absolutely necessary," a philosophy rooted in the pressure to maintain schedules on one of the world's most heavily used rail networks. Mikawashima exposed the lethal cost of that thinking. In the aftermath, JNR reversed the policy entirely, adopting the principle: "Stop trains as soon as an accident happens, regardless of its scale." By April 1966, an Automatic Train Stop system had been installed across all JNR lines, designed to halt any train that passed a red signal. The disaster at Mikawashima did not just change a rule; it rewired the philosophy of Japanese rail safety. Today, Japan's railways are among the safest in the world, a reputation built in part on the hard lessons of that May evening in Arakawa.