Custer Creek itself normally runs dry for nine months of the year and had never been known to rise to a depth of more than. But on the night in question a cloudburst deposited an estimated of rain on the area drained by the creek. The previous train had crossed the bridge at 10:15 p.m. at which point the engineer estimated the water to be about deep. Twenty minutes later, in view of the heavy rainfall experienced the section foreman performed an inspection of the track and estimated the depth of water to be beneath the level of the girders of the bridge, giving no indication of the trouble to come.
''Olympian''
The westbound Olympian that night was hauled by 4-8-4 No.220 and comprised eleven cars. It was traveling from Chicago to Tacoma and carrying 155 passengers when it neared Custer Creek at a speed of 50 mph. There was no water on the track to warn the engineer that beneath was a torrent of water high, battering at the bridge foundations, and no brake application was made.
Wreck
As the Olympian crossed at 12:35 a.m. the bridge collapsed; the engine and seven passenger cars were thrown into the swollen creek On the west bank the locomotive and five cars were "piled in a shambles of crumpled steel", killing the engineer and fireman. The wreck happened so quickly that when the body of the engineer was recovered he was still sitting in his seat with his hand on the throttle. "Two other cars ended up deep in the roaring creek". Rescue efforts were mounted by the train crew and uninjured passengers; smashing windows on the partly submerged cars to provide escape routes. Although the official death toll stands at 47, this is an estimate as several bodies were swept into the Yellowstone River, one body being recovered at Glendivedownstream. 75 people were injured. Newspapers reported the paradox that modern air-conditioned rolling stock requiring sealed windows, and the use of shatterproof glass was partially responsible for some deaths
Investigation
The investigation determined that the volume and velocity of water flowing beneath the bridge that night was "much in excess of any that had been experienced before or might be anticipated at this place". The bridge structure was still intact when the train reached it but two of the central piers had been undermined. The weight of the locomotive caused the piers to subside and the bridge to collapse.