The collision occurred on February 22, 1975, in a valley where the Dovre Line runs along a single track between Oslo and Trondheim. Both trains carried roughly 400 passengers each, nearly all of them heading to or from skiing destinations along the route. The scale of the occupancy made the crash catastrophic: approximately 800 people were aboard when the two trains struck. Most of the dead were concentrated in a single car of the northbound train, where the force of impact compressed the carriage. Twenty-five more people were injured. Among the dead was Tonnes Andenaes, a Norwegian politician, and one American citizen. The operators of both trains survived the collision. One of them had jumped from his cab before impact.

The cause was devastatingly simple. Train 351 was running late. Under the line's operating procedures, the northbound train was supposed to stop at Tretten station and wait there while the southbound express passed on the single track. Instead, it continued north beyond the station, entering the stretch of single track where the express was approaching from the opposite direction. A station master recognized the danger. He cut electrical power to the tracks and attempted to warn both trains, but his intervention came too late. The trains were already closing on each other at combined speed, with insufficient distance remaining to stop. On a single-track railway, the margin between routine and disaster is a single missed protocol.

The Gudbrandsdalen valley, where the collision occurred, is one of Norway's great north-south corridors, a glacier-carved passage linking the country's two largest cities. The Dovre Line follows the valley for much of its length, threading through small towns and ski stations that fill with holiday travelers every winter. Tretten is one of these communities -- a village in the valley where the station served as a passing point for trains traveling in opposite directions. The February timing meant short daylight, cold temperatures, and a landscape deep in snow. Rescue operations in the valley's winter conditions added difficulty to an already overwhelming scene. The wreckage of two full-length trains, scattered along the track a kilometer north of the station, confronted responders with destruction on a scale Norway had not experienced on its railways since the Second World War.

The Tretten disaster prompted a national reckoning with railway safety in Norway. The crash exposed vulnerabilities in the single-track system that depended on human compliance at each passing station -- a system where one delayed train and one missed stop could produce exactly the outcome that occurred on February 22. The human cost was concentrated and intimate: twenty-seven dead in a country where the railway community was small and the victims were neighbors, families on holiday, children traveling with their parents. The disaster remains the benchmark against which Norwegian rail safety is measured, a reference point in every subsequent discussion about track signals, passing protocols, and the risks inherent in single-track operations. The valley through which those two trains traveled still carries the Dovre Line, still carries skiers, and still passes through Tretten.