The high plains of eastern Montana are prone to violent weather, but what descended on the Custer Creek drainage that night exceeded all recorded experience. A cloudburst dumped massive amounts of rain on the watershed in a concentrated period. The creek, normally a dusty scar across the landscape, transformed into something its banks could not contain. At 10:15 p.m., a train had crossed Bridge AA-438 with the engineer estimating the water at a moderate depth. By the time the Olympian approached two hours later, an invisible flood was battering the bridge foundations with a volume and velocity the Interstate Commerce Commission investigation would later describe as much in excess of any that had been experienced before or might be anticipated at this place. The bridge structure was intact as the locomotive reached it. But two central piers had been undermined. The weight of the engine caused them to subside, and the entire structure collapsed.

The Olympian was a modern train, one of the Milwaukee Road's flagship passenger services on its transcontinental route. It carried 155 passengers in eleven cars, including air-conditioned coaches with sealed windows and shatterproof glass, the latest in comfort and safety engineering. When the bridge gave way, these features became death traps. The engine and seven cars plunged into the swollen creek, with two more ending up deep in the roaring water. Passengers trapped in partly submerged cars could not break the windows designed to withstand impact. Newspapers reported the bitter paradox: the very technologies meant to protect passengers had contributed to the death toll. Rescue efforts began immediately as train crew and uninjured passengers smashed through the shatterproof glass to create escape routes for those trapped inside.

The official count stands at forty-seven dead, though some contemporary sources reported as many as forty-nine. Seventy-five passengers suffered injuries. For Montana, it remains the worst rail disaster in state history. The victims had boarded in Chicago, bound for Tacoma and points along the way, ordinary travelers caught in a convergence of weather and infrastructure that no one had predicted. The investigation would find no fault with the railroad's maintenance or inspection practices. The bridge had been sound. The riprap had been adequate for any previously recorded conditions. What killed the passengers of the Olympian was the simple impossibility of anticipating a flood that had never happened before. The creek that ran dry most of the year had become, for a few terrible hours, something more powerful than the engineering designed to cross it.

Near Saugus, Montana, southwest of Terry, the Milwaukee Road rebuilt and continued operating for decades before the company's 1980 bankruptcy ended passenger service across these plains. The Olympian Hiawatha, successor to the original Olympian, made its last run in 1961. Custer Creek returned to its seasonal pattern, dry most months, occasionally rising after storms, never again matching the fury of that June night. The site carries no grand memorial, just the quiet truth that infrastructure built for reasonable conditions can be overwhelmed by unreasonable events. The investigation recommended better warning systems for washouts, a lesson learned at terrible cost. For those who fly over this stretch of Montana, the land looks empty and predictable, brown grassland cut by minor drainages. But the names on the map remember what once happened when the sky opened and the water rose: Custer Creek, where forty-seven journeys ended in a flood that came from nowhere.