Base tunnel

This type of tunnel typically connects two valleys at about the same altitudes.

[1] When originally constructed, classical railway lines through mountainous terrain tried to minimize tunneling, due to technical limitations and expense, and therefore required long and steep gradients and many curves, or even spirals.

The base tunnels take the opposite approach, minimizing or eliminating gradients and curves with the consequence of having longer tunnels but shorter total distances to travel.

This allows for higher speeds and lower energy costs.

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