The previous day had been triumphant. On 17 September, King Haakon VII and a retinue of politicians, engineers, and dignitaries had ridden the inaugural train of the new Dovre Line, Norway's most ambitious railway project. The official opening ceremony took place at Hjerkinn Station, the highest point on the line, and between there and Trondheim the king's train stopped at every station so Haakon could greet the crowds that had gathered along the route. The train arrived in Trondheim that evening to jubilant celebrations, and the festivities continued through Sunday. Most of the distinguished guests, however, had duties waiting in Kristiania - the capital not yet renamed Oslo. An extra night train was arranged to take them home: six sleeping cars sandwiched between a luggage van and a conductor's car, pulled by two NSB Class 30b locomotives. Ninety-six passengers boarded.

The extra train, designated Litra D, was originally scheduled to depart Trondheim at midnight, safely after the arrival of northbound train no. 361 from Storen. But someone decided to push the departure fifteen minutes earlier, to 23:45, intending that the two trains would meet and pass at Marienborg station. Litra D left nine minutes late, at 23:54, and received an all-clear signal through Skansen. At Marienborg - little more than a short passing loop with a guard's cabin - a single man named Peter Wiig was responsible for managing the meeting. Usually two men handled such encounters. Wiig set the points correctly and signaled no. 361 into the passing loop. He used his handheld lamp to guide the approaching train safely in, afraid that in the darkness the crew might miss the fixed signals and stop prematurely. Train no. 361 slowed as it entered Marienborg. Then, to Wiig's horror, it kept going - through the station, past the stop signal, and out the northern end onto the main line.

Multiple safeguards failed in the same moment. The fireman on no. 361, who should have been watching for signals on the right side, was distracted by a problem with the engine's lubricator. The train manager in the first carriage was supposed to watch for a hand signal from the station guard, but he was struggling to open a stuck window and missed the fact that no guardsman was standing there. The driver felt a jolt as the train crossed the misaligned points leaving the station and slowed, puzzled, trying to understand what had happened. Wiig grabbed his telephone and called Skansen, 900 meters to the north, ordering them to stop Litra D. But the extra train had already passed. At 23:57, the driver of no. 361 saw lights emerging from the tunnel ahead. The driver of Litra D saw the oncoming headlight only seconds before impact. Both trains braked. It was not enough. The locomotives survived largely intact, but behind Litra D's engines the second and third sleeping cars crushed into each other and tipped over, absorbing most of the collision energy.

All six dead were prominent figures who had attended the Dovre Line inauguration. Among them were Thomas Heftye, a former Liberal Minister of Defence; Erik Glosimodt, the railway architect who had designed the very station building at Hjerkinn where the line was officially opened; Captain Thoralf Bjornstad of the Norwegian State Railways board; and Nils Johannes Sejersted, director of Norway's official mapping agency. King Haakon, still in Trondheim, visited the crash site that night and remained until half past four in the morning. Four men were charged: the train manager, engineer, and fireman of no. 361, plus Peter Wiig. All four were acquitted in February 1922. The court accepted that the scheduling phrase "night to Monday" was genuinely ambiguous in 1921 and had never before been applied to a train departing before midnight. The defendants returned to railway work. Wiig was later promoted to station master. The disaster led to sweeping safety reforms - mandatory crew conferences before departure, clearer scheduling language, and enhanced signaling at Marienborg, implemented by 1926. Many of the crash survivors were members of the Stortinget, Norway's parliament, and they proved more than willing to fund the improvements.