The Chicago, Rock Island and Pacific Railroad (CRI&P) is connected to the Chicago, Burlington and Quincy Railroad (CB&Q) at Ottawa, Ill., which is on the Streator Branch 41 miles west of Montgomery. A few days before the accident, CRI&P trains started detouring over the CB&Q tracks between Ottawa and Chicago because of a defective CRI&P bridge over the Des Plaines River at Joliet, Ill. Some time before 8:45 p.m. on the day of the accident, CRI&P passenger trains No.4 and No. 10 were combined at Ottawa to detour eastward over the CB&Q to Chicago, via Montgomery interlocking. This combined train, consisting of six CRI&P diesel-electric units and 19 cars, operated on the CB&Q as Extra RI 656 East. It left Ottawa at 8:45 p.m. with a CRI&P engine crew, a CB&Q engineer-pilot, and a CB&Q road foreman of engines in the control compartment at the front of the locomotive. A CB&Q conductor-pilot and a CRI&P train crew were at various locations in the cars.

Approximately 2 hours after leaving Ottawa, Extra RI 656 East arrived at Montgomery interlocking, where it stopped on the Streator Branch main track with the front end 675 feet west of switch 14 and 195 feet west of signal 24-23, which indicated Stop. About the same time, the men on the locomotive saw a detouring westbound CRI&P passenger train, Extra RI 634 West, stopped on track No. 1 east of the interlocking station. A few minutes later, they saw the headlight of No.3, a west-bound CB&Q passenger train, approaching on track No.2 and surmised that their train would be routed eastward on track No.2 after No. 3 passed. While Extra RI 656 East was waiting for No.3 to pass, a warning device sounded in the control compartment of the first diesel-electric unit and the CRI&P fireman went into the engine room of this unit to determine why the warning device had sounded. Immediately after the fireman left the control compartment, the CB&Q road foreman of engines noticed No.3 had been diverted to the Streator Branch main track at switch 14 and was closely approaching at high speed. He promptly called a warning to the CRI&P and CB&Q engineers and ran to a side door of the control compartment, where he started to jump from the locomotive. Before he could jump, however, No.3 struck the front end of Extra RI 656 East, killing the CRI&P engineer and the CB&Q engineer-pilot. The CB&Q road foreman of engines and the CRI&P fireman were injured.

Number 3, a westbound first-class passenger train, consisting of 3 diesel-electric units and 15 cars, left Aurora at 10:45 p.m., 3 minutes late, and proceeded westward on track No.2. A few minutes later, it approached Montgomery interlocking at 63 miles per hour, as indicated by the speed recording tape. The engineer and fireman were in the control compartment at the front of the locomotive, and the other crew members were at various locations in the cars. Signal 5-3 indicated Proceed as the train approached Montgomery interlocking, and this indicated to the enginemen that the route was lined for movement of No.3 through the interlocking on track No.2. The route, however was improperly lined for movement of No.3 from track No.2 to the Streator Branch main track, via switch 14. The engineer apparently first became aware of this when the train reached the area of the interlocking station at which time he initiated an emergency brake application. A few moments later, the train passed the interlocking station, entered the Streator Branch main track at switch 14 and, while moving at 52 miles per hour, struck the front end of Extra RI 656 East.