Storm Alfrida reached Denmark on 1 January 2019, and overnight the Danish Road Directorate closed the Great Belt Bridge to all road traffic, expecting to reopen at eleven the next morning. Rail traffic was not restricted, and the reasoning was sound. The eighteen-kilometre link between Zealand and Funen is not one crossing but two. Road and rail share the West Bridge, a low box-girder structure close to the water, while the East Bridge — the great suspension span that appears in every photograph of Denmark — carries only cars, because trains cross that stretch through a tunnel beneath the seabed. The trains were on the sheltered side. Nobody had reason to stop them.

Coming the other way was a DB Cargo freight train, crewed by one man, loaded with semi-trailers full of empty Carlsberg bottle crates. The two met between the islet of Sprogø and the Funen shore. The driver of the passenger train said afterwards that he had seen sparks in the wrong place — lower than they should have been, not up at the overhead catenary — and that something big was hanging from the freight train coming toward him. The object was initially thought to be a tarpaulin. It was a semi-trailer, swinging free from its wagon, and with two trains closing at line speed there was no interval in which anyone could act.

Eight passengers were killed: five women and three men, between twenty-seven and sixty years old, all Danish citizens, all identified by 4 January. Sixteen others were injured, none critically — fourteen slightly, two moderately. The storm and the setting made the response brutally difficult; the trains were stopped mid-span on an exposed bridge in a gale, with the road shut. Six deaths were confirmed while the trains were still out there. After they had been moved to a secluded area, two more bodies were found in the passenger carriages. The National Operative Staff, the government's crisis coordination body, was convened. The bridge reopened to road traffic at around 12:20 that same day, and the westbound carriageway had to be closed again briefly that afternoon because drivers were slowing to look. Rail traffic resumed at 10:40 on 3 January on a single track, and ran normally by that evening. It was Denmark's deadliest rail accident since 1988.

The Accident Investigation Board Denmark took the case. On the day, investigator Bo Haaning said the crash was a collision between the passenger train and an empty semi-trailer from the freight train, while staying careful about the sequence: it either hit the train, or the train drove into it. Public attention went first to the weather. Gusts of up to 20.9 m/s — about 47 mph — had been measured, with sustained winds lower, blowing from the north, straight across the line of the bridge. Banedanmark, which runs the railway, and Sund & Bælt, which runs the crossing, both stated that every procedure had been followed. They were telling the truth, and that was the uncomfortable part. Attention shifted to the wagon itself. On a pocket wagon, the front of a semi-trailer is held by nothing but its kingpin; the rear stays put only because the wheels sit down into a recess. DB Cargo suspended its bottle-crate services. The accident resembled an incident at Hamburg, where an inadequately secured semi-trailer had come loose and struck a bridge pier. The European Union Agency for Railways is supposed to circulate information about serious incidents. Danish authorities had never been told.

The measures came quickly. On 5 January the Danish Transport Authority issued a warning about pocket wagons on the investigation board's recommendation, stating that the locking mechanism securing a trailer could pose a safety risk, and asked the Joint Network Secretariat to open a formal procedure. On 7 January a test at DB Cargo found that a semi-trailer could still be lifted off a wagon after being loaded and secured — and this train had been checked before departure and had appeared properly secured. The following day the authority temporarily banned semi-trailers on pocket wagons until additional locking procedures were in place, and tightened the wind limits for freight crossing the Great Belt. Comparable measures soon spread across the continent. The final report came on 18 December 2019. The semi-trailer had not been locked in place, and the reason the lock failed was improper maintenance: the mechanism requires regular lubrication, the official guidelines for doing so were incomplete, and parts of it went insufficiently greased. Underneath that lay a question nobody had answered: whether maintaining it fell to DB, which leased and operated the wagon, or VTG Rail, which owned it. Had the lock worked, the wind could not have pulled the trailer free. One smaller, sadder postscript: a DB Cargo union representative crossed out the name of the colleague who had inspected the loaded freight train before departure. He received a 20-day suspended sentence for evidence tampering, upheld on appeal.