Eighteen kilometres under the Baltic Sea, connecting Denmark and Germany, and when it opens it will be the closest thing to the Channel Tunnel anywhere in the world. This is the story so far, why it matters for rail travel from the UK, and when you might actually be able to use it.
The Fehmarn Belt is a strait in the western Baltic Sea, about 18 kilometres wide, separating the Danish island of Lolland from the German island of Fehmarn. At present, crossing it means either a 45-minute ferry between Rødbyhavn and Puttgarden, or a 160-kilometre road detour westward via the Great Belt bridges. Neither is fast, and for rail passengers the bottleneck has been even worse — scheduled passenger trains stopped using the ferry route in December 2019, pushing Hamburg-to-Copenhagen journeys the long way round and adding time rather than saving it.
The Fehmarn Belt Fixed Link, under construction since 2020, is the solution. It's an 18-kilometre tunnel under the strait, combining a double-track electrified railway (capable of 200 km/h) with a four-lane motorway, separated inside the tunnel and sharing a central service tube. When it opens, the crossing will take seven minutes by train and ten by car. The 45-minute ferry — run by Scandlines on what was historically called the Vogelfluglinie or "bird flight line" — will be replaced by a tunnel that never stops running.
This is where the Fehmarn Belt and the Channel Tunnel part company. The Channel Tunnel was bored — eleven enormous tunnel boring machines chewed through the chalk marl under the seabed between 1988 and 1994. The Fehmarn Belt tunnel isn't being bored. It's being immersed, which is a fundamentally different engineering approach.
Immersed-tunnel construction works like this. On land, at a purpose-built factory on the Danish coast at Rødbyhavn, the tunnel is cast in sections — huge rectangular concrete boxes called elements. Each standard element is 217 metres long, weighs around 73,500 tonnes, and contains all five tubes (two rail, two road, one service) in its final cross-section. The Fehmarn Belt tunnel will consist of 79 standard elements plus 10 shorter special elements, for 89 in total. Each is watertight, fitted out internally, and sealed at both ends.
Separately, a trench is dredged across the seabed along the tunnel's planned route. In the Fehmarn Belt this trench is about 40 metres deep and 18 kilometres long, and required dredging roughly 15 million cubic metres of material — much of which has been reused to create new coastal nature reserves on the Danish side.
When both are ready, each element is towed out into the strait by tugs and lowered into place on the seabed trench, aligned with the previous element, and connected end-to-end under water. After all 89 are in place, the trench is backfilled, the internal systems are commissioned, and the tunnel portals and ramps connect it to the rail and road networks at each end. The whole thing, when finished, sits on the seabed rather than tunnelled through rock beneath it.
Why immersed rather than bored? The Fehmarn Belt is shallower than the English Channel and the geology is softer — the seabed is mostly glacial clay and silt rather than the chalk that the Channel Tunnel passes through. For those conditions, immersed construction is typically cheaper and technically lower-risk than boring. The trade-off is that immersed tunnels need extensive marine work, which depends on weather windows and specialised vessels, and these have proven to be the single biggest source of delay (more on that below).
At 18 km, the Fehmarn Belt will be the world's longest immersed tunnel — comfortably past the previous record-holders — and also the world's longest combined road-and-rail tunnel of any kind. It doesn't challenge the Channel Tunnel's 50 km length record (which is a bored tunnel), but within its category of construction it sets a new mark.
The headline change is Copenhagen to Hamburg. Currently, that journey takes around four and a half to five hours by train, routed the long way round via Odense and the Great Belt Fixed Link. Once the Fehmarn Belt tunnel opens and the connecting rail lines are upgraded to 200 km/h on both sides, Copenhagen to Hamburg will drop to around two and a half hours — genuinely competitive with flying once airport transfers are included.
Further north, Copenhagen already connects directly to Stockholm via the Øresund Bridge-tunnel (which opened in 2000) and onward Swedish high-speed rail. Further south, Hamburg is a major German rail hub with direct ICE services to Berlin, Frankfurt, Munich, and westward to the Netherlands and Belgium. The Fehmarn Belt link closes the last significant gap in the Scandinavian-Mediterranean corridor — the EU's priority rail axis running from the Finnish border to Malta — and transforms what has historically been a slow, multi-change journey into one high-speed spine.
For freight, the implications are arguably even bigger. Trucks and freight trains between Scandinavia and continental Europe currently detour 160 kilometres west. The new link cuts that entirely, which is why the German and Danish governments, the EU's Connecting Europe Facility (contributing over €1 billion), and the Trans-European Transport Network have all backed the project with substantial funding.
Today, travelling from London to Copenhagen entirely by rail is possible but slow — around 14 to 15 hours with two or three changes, typically routed London → Brussels → Cologne → Hamburg → Copenhagen, with the Hamburg → Copenhagen leg being the bottleneck. It's effectively an overnight journey and in practice most people fly.
Once the Fehmarn Belt tunnel opens, that same journey compresses substantially. A reasonable projection: London → Brussels in around two hours, Brussels → Cologne in about 90 minutes, Cologne → Hamburg in around three and a half hours, and then Hamburg → Copenhagen in two and a half. That's a roughly nine-and-a-half-hour day by rail from London to Copenhagen, with connections already possible today on the German and Belgian legs. For Stockholm, add another five hours on Swedish high-speed rail north from Copenhagen. For Oslo, a Norwegian connection from Göteborg.
It also changes the calculus on sleeper trains. The overnight services between continental Europe and Scandinavia have had a rough few decades — the Berlin-Malmö route ran via Sassnitz and a train ferry until 2019, was cancelled during the pandemic, then was reinstated on the longer inland Jutland route. When the Fehmarn Belt opens, a true high-speed overnight London → Scandinavia route becomes feasible for the first time — whether via revived sleeper services or a fast day-and-night combination starting with a same-day afternoon departure from St Pancras.
The two projects make an illuminating comparison. They are the only two major fixed rail links crossing open sea in Europe, and they'll shortly be the only two in the world on this scale. But they're different in almost every technical particular:
The Channel Tunnel is a set of three bored tubes (two single-track rail, one service) dug through chalk marl an average of 45 metres below the seabed. It carries passenger trains and a separate vehicle shuttle service — cars and lorries travel on carriages, not under their own power. It opened in 1994 and took £4.65 billion to build (around £12 billion in today's money), with no public subsidy — the project was financed privately and its original backers famously lost most of their money before the service became profitable.
The Fehmarn Belt is five tubes (two rail, two road, one service) in a single immersed box structure, around 40 metres below the sea surface. Critically, cars and lorries drive through the tunnel under their own power — there's no shuttle involved. The project is funded by the Danish state through user charges, with German contributions for the mainland motorway connection and substantial EU support. Total cost is expected to exceed €10 billion once all hinterland upgrades are included.
One more difference worth noting: the Channel Tunnel is a rail-only tunnel (cars travel on the LeShuttle trains but don't drive through a road tunnel). The Fehmarn Belt is both. Anyone who's taken LeShuttle will appreciate the difference — on the Fehmarn Belt, you'll simply drive straight through.
Construction officially began on the Danish side on 1 January 2020 and on the German side in 2021. The Rødbyhavn factory on Lolland is one of the largest construction sites in Europe — it covers the equivalent of 300 football pitches and employs more than 2,000 people from 40 countries. As of early 2026, most of the standard tunnel elements have been cast, the 18-kilometre seabed trench was completed in 2024, and the tunnel portals on both sides are in the final phases of completion.
The original opening date was 2029. In October 2025, however, the Danish state-owned builder Sund & Bælt announced that the specialised immersion vessel — called IVY, purpose-built for this project and not yet fully tested by the Danish Maritime Authority — was running about 18 months late, which would delay completion of the tunnel itself by at least two years. Announcements since suggest an opening around 2031, though this is not yet fully confirmed. Crucially, the Danish rail upgrade works are on schedule — it's the marine-immersion phase that's the critical path.
More positively: IVY was finally approved for operation in early 2026, and the first tunnel element is expected to be immersed during April 2026. Once immersion begins in earnest, the pace should pick up significantly — 89 elements is a serious but well-understood sequence of operations.
If you're thinking about a train trip from the UK to Scandinavia and the question is "should I wait for the Fehmarn Belt tunnel?", the honest answer is: probably not, unless you've got flexibility around 2031 or 2032. The tunnel will transform journeys once open, but the current multi-hour routing via Jutland still works, and the overnight trains from Hamburg to Copenhagen continue to run.
The Fehmarn Belt is the most visible piece of a wider push to fix Europe's rail geography — the idea that you should be able to move between major European cities at competitive speeds without changing operators, stations, or modes. The Channel Tunnel did that for the UK-to-continent corridor in 1994. The Swiss Gotthard Base Tunnel (2016) and Ceneri Base Tunnel (2020) did it for the north-south Alpine crossings. The Fehmarn Belt will do it for the north-south Scandinavia-to-central-Europe corridor. Further projects are on the horizon — most notably the Brenner Base Tunnel between Austria and Italy, due to open 2032 — but Fehmarn Belt is the next major completion.
For a traveller, the cumulative effect is significant. Within a decade, the proposition "from anywhere in Europe to anywhere else by train, in a day or an overnight" becomes broadly true. The Channel Tunnel is already the UK's entry to that network. The Fehmarn Belt is the Baltic's missing link.