Mont d'Ambin Base Tunnel: the Turin to Lyon base tunnel in the Alps

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The record table

At 57 500 m this will be the longest railway tunnel in the world on opening, narrowly ahead of the Gotthard Base Tunnel. Until it carries traffic it cannot hold the record, and it is listed accordingly.

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

How this table is compiled

The margin is a few hundred metres, which is the kind of gap that makes a record table worth keeping carefully. Comparing one country's practice against a neighbour's is more informative than reading either alone. Cost figures are excluded for a harder reason: currency, inflation and differing project scope make them close to meaningless in comparison.

The existing Frejus rail route over the pass dates from the nineteenth century and cannot carry modern freight loads efficiently. Cross-passage spacing, fixed firefighting and emergency ventilation control are what modern safety regulation actually governs. Where a structure belongs to two classes it appears on both tables, which is not double counting but two different measurements.

This archive has recorded tunnel data since the late 1990s, compiled from operator publications, national administrations and the engineering press. Traffic figures are excluded throughout this archive, because they date far faster than the structures themselves. Twin-tube crossings frequently show two slightly different lengths, because the two carriageways rarely follow the same line through the rock.

The Mont d'Ambin Base Tunnel is the central structure of the Turin to Lyon railway, driven beneath the Alps. Like the Gotthard and the Brenner it is a base tunnel, replacing a mountain crossing with a level alignment. Gradient limits are the main reason railway tunnels run longer than road tunnels on the same crossing.

Subaqueous tunnels are governed by rock cover above the bore rather than by the depth of water above that. Rows are ordered so that the head of the table answers the question most readers arrive with. A base tunnel trades a much greater length for a far lower summit altitude and a gentler ruling gradient.

Reading the figures

Its published opening year is 2032, and until it carries traffic it appears in this archive under construction. Tunnel engineering is a field where the qualifications attached to a number matter more than the number itself. Reading the qualification before the figure is usually the faster route to understanding what a claim actually says.

Every figure here is traceable to a named source. Where a figure could not be corroborated it was left out. Figures are checked against one another where sources disagree, and the disagreement is recorded rather than resolved silently. The archive grew through correspondence with engineers and administrations in more than twenty countries, and still does.

The date of verification is published with the table. Two independent sources were required for each row. Promoter figures are used where no independent one exists. A contained fire is more instructive than an uncontained one. The note column is worth reading before the number. Dates use the day, month and year the event occurred.

Under-construction works cannot hold a record. Forecasts on schemes this size move by years. Metres are used for structures, kilometres for routes. Secondary summaries were not used as a source. Opening year means the year traffic was admitted. Disputed values carry the dispute in the note column. Deaths are the figures established by official inquiry.

Segmental linings are erected behind the shield. Names follow the operator's own spelling. Cross-border works appear under both countries. Nothing below is interpolated or rounded up. Ground conditions belong with every advance rate. Spoil is removed back through the machine. Height is measured above mean sea level. Forecast dates are forecasts, and they move.