At their core, the vast majority of transportation tunnels are designed to help relieve capacity, either by adding additional road networks, shortening the time taken to traverse congested areas or by implementing rail, tram or metro alternatives.

All of these different solutions to what are often very similar problems rely on tunnel boring and conveyor belt installations to move vast amounts of earth away from the site.

Generally, there is a singular purpose to constructing a tunnel, given the time, finances and labour involved in such a grand project, with some secondary considerations factored into the design.

However, there are a surprising number of double-deck tunnels that have been constructed over the years, which consist of two floors and often more than one purpose.

They are typically developed as somewhat major solutions to major issues, and by exploring how just a few of them are constructed, it becomes clear what they can and cannot assist with.

The Double-Deck That Failed

Arguably the very first double-deck tunnel in existence is the Queensway Tunnel in Liverpool was at one point described as the “eighth wonder of the world”, costing £8m in 1934 (£478m adjusted for inflation) and still the longest road tunnel in the United Kingdom.

The need for the “old tunnel” had been around for a century, as whilst the ferry across the Mersey had been romanticised by Gerry and the Pacemakers, the congestion and queues for the ferry, when it was the only way across the River Mersey for cars, made it unbearable at times.

The tunnel was constructed without the use of a TBM,  by slowly excavating 1.2m tons of clay, rock and gravel with the help of 1700 workers.

It was designed to be a double-deck tunnel, with not only four lanes of traffic but also a tramway that operated on the lower part of the tunnel, which would have provided a rapid transit system across the river.

This did not happen, largely due to bureaucratic wrangling; the funding in Birkenhead necessary to help meet the ballooning was secured on the strict condition that trams would never use it, which to this day has remained the case.

Yerba Buena Tunnel

The largest-bore road tunnel in the world, the Yerba Buena Tunnel was a phenomenal feat of engineering built as a complement to another incredible engineering wonder, the San Francisco-Oakland Bay Bridge.

Both of these engineering marvels were designed to have two decks; one would be exclusively for road use, whilst the other would have a railway link and three additional lanes of road.

Two decades later, this would be refined, and to this day both the upper and lower decks are for road use, making it an unusual example of a double-deck tunnel (and bridge) exclusively for the purpose of a single mode of transportation.

The reasons for this change are somewhat murky; whilst ridership had indeed reduced, there were claims that National City Lines ran a transportation monopoly and unfairly used its competitive advantage to force Key System (who ran the Bay Bridge and Yerba Buena Tunnel trains) and others out of business.

National City Lines was indicted on two charges of conspiracy to monopolise and was acquitted of one and convicted of the other.

By 1958, no trains ran across the route and the tracks were converted to more roads.