The German Vogelfluglinie and the Danish Fugleflugtslinjen mean the same thing: bird flight line. It is not marketing. Cranes and Arctic waterbirds migrating between central Europe and Scandinavia funnel along almost exactly this corridor, using the shortest hop over water between the German coast and the Danish islands, and the engineers who wanted to shorten the Hamburg-Copenhagen journey wanted the same line for the same reason. Rail and road here follow a flight path that was in use long before anyone drew a timetable.

The architect Gustav Kröhnke put forward plans for a direct Hamburg-Copenhagen link in 1863; the Second Schleswig War of 1864 ended that conversation. A German-Danish committee to promote the Fehmarn route formed in 1910 and got nowhere. Then came the version nobody celebrates. After Germany occupied Denmark in 1940 the architect Heinrich Bartmann produced construction-ready designs, and in September 1941 the Organisation Todt started throwing up embankments and a small autobahn bridge, using the leverage of occupation to move a project that fifty years of negotiation had not. The war stopped the work in 1943. Afterwards, partition made things worse before they got better: Warnemünde ended up in East Germany, so the shortest existing route between Copenhagen and Hamburg ran through the wrong country. A stopgap ferry from Gedser to Großenbrode carried the traffic from 1951 to 1963.

Construction restarted in earnest in early 1958, on the bridge over the Fehmarnsund and on the ports at either end of the crossing. The Fehmarnsundbrücke opened on 30 April 1963: 963 metres long, a steel network arch spanning 248.5 metres, with 22 to 23 metres of clearance for shipping and room on the deck for a road, a railway and a combined foot and cycle path. Its silhouette earned it the nickname Kleiderbügel - the coat hanger - and it has been a landmark of Schleswig-Holstein ever since, protected as a cultural monument since 1999 for its technical, scientific and artistic significance. Two weeks later, on 14 May 1963, the whole line was inaugurated with President Heinrich Lübke and King Frederik IX in attendance, one hundred years after Kröhnke's drawings.

The heart of the line is water. Nineteen kilometres separate Puttgarden on Fehmarn from Rødby on Lolland, and ferries have shuttled between them since 1963, originally run jointly by the German and Danish state railways and now by Scandlines. The crossing took an hour until double-ended ships arrived in 1997 and cut it to forty-five minutes. In peak summer the operation runs up to 48 round trips a day - 96 crossings of the belt - around the clock. Landside, the road is European route E47, becoming the A1 autobahn in Germany, though the last stretch to Puttgarden is still a two-lane Bundesstraße, the 207.

The rail side unravelled first. Only some Danish IC3 trains were ever equipped to run into Germany, and the German ICE TD units used on the route went out of service in 2017. When the Great Belt Bridge opened, freight abandoned the ferry entirely. Since December 2019 all Hamburg-Copenhagen passenger trains have gone the long way around, via Funen and Jutland - 160 kilometres further but roughly twenty minutes quicker, and long enough to run proper trains. Rail service on the German side shrank next: from 31 August 2022 the stretch between Neustadt and Puttgarden was handed over to buses. What remains is a road corridor with a ferry in the middle of it, waiting for the Fehmarn Belt tunnel to restore the direct rail link that the bird flight line was drawn for in the first place.