Base tunnel

A base tunnel is a type of tunnel, mainly a railway tunnel, that is built through the base of a mountain pass. This type of tunnel typically connects two valleys at about the same altitudes.

When originally constructed, classical railway lines through mountainous terrain tried to minimize tunnelling, due to technical limitations and also expense, and therefore required long and steep gradients and many curves, or even spirals. Such tunnels are sometimes also called culmination tunnels, especially in the presence of a base tunnel through the same mountain massif.

The base tunnels take the opposite approach, eliminating, or at least, minimizing gradients and eliminating curves with the consequence of having longer tunnels on a route, but shorter total distances to travel, allowing higher speeds and lower energy costs. Usually, there is an already existing culmination tunnel, and it would not have been possible or reasonable to build a tunnel at an even lower altitude.

Some of the best-known base tunnels are (length and opening):

Base tunnels through the San Gabriel Mountains have been proposed as part of the California High Speed Rail system.

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