The race for the longest tunnel: every record holder since 1871

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The succession

Each step is the year a new longest tunnel opened in its class. A record can stand for decades: Simplon II held the railway record from 1922 to 1982.

How this archive measures: lengths, bores, dates and ordering

How this table is compiled

The railway record has changed 7 times since 1871 and the road record 4 times, on the figures recorded here. Ventilation arrangement caps how long a bore can be driven more directly than money does. Tunnels are regularly repurposed, and the mode a bore was built for is not always the mode it now carries.

Smoke moves uphill, so the two portals of a burning tunnel are not equally dangerous. Cross-border schemes require two states to fund the same hole at the same time, which is why they take decades. Nothing in either row is estimated: a value that could not be corroborated is left blank rather than filled in.

Cross-passage spacing, fixed firefighting and emergency ventilation control are what modern tunnel safety regulation actually governs. A tunnel's gradient is often the more interesting figure, particularly on subsea crossings where vehicles have to climb back out. The interesting figure on most crossings is the gradient or the cover, and neither appears in a ranking by length.

Rock cover above the bore governs a subsea design, not the depth of water above that. Overburden at the deepest point sets the lining, and on a long crossing it varies more than the length suggests. Adding a metre to the longest tunnel in the world costs what adding a kilometre to an ordinary one does.

A national administration publishes what it maintains, so a crossing that changes owner can move between lists. Pumps run for the life of a subaqueous crossing and are the one system that is never switched off. This archive has held per-country and per-class tunnel inventories since the late 1990s, compiled from operator and administration sources.

Reading the figures

Wartime damage closed several crossings for years and a few were never reopened to their original use. Two crossings of the same length built fifty years apart are different objects and only the number is comparable. Gradient limits are the main reason a railway crossing runs longer than a road crossing of the same range.

Simplon II held the railway record for sixty years, which is the longest unbroken hold in this archive. Opening dates refer to the admission of traffic, not to breakthrough, which normally falls a year or two earlier. Lighting is brightest at a portal in daylight, because the adapting eye is the hazard rather than the dark.

Compressed-air working made shafts through wet ground possible at a cost paid by the men who sank them. Traffic volumes are excluded for the same reason, and because they date far faster than the structures themselves. A base tunnel trades much greater length for a far lower summit altitude, which changes what the number means.

Comparing two national records says as much about how each country documents its infrastructure as about the infrastructure. Motorway crossings are usually twinned, and the second bore is often driven decades after the first. Catenary needs headroom a road tunnel does not, so an electrified bore is taller for the same traffic.

Height above sea level is a separate record from length. Depth below sea level is recorded where published. The longer bore is not the harder job. Salt water attacks reinforcement differently from fresh, and subsea linings are specified accordingly. Portal design follows avalanche and rockfall exposure rather than architectural intent.