Historically, the Khun Tan range was a formidable natural barrier between the Lanna Kingdom and the Siamese in the central plain. So the Khun Tan tunnel (as well as the Khao Phleung tunnel near Uttaradit) was an important development for securing the northern reaches of Siam.
Its Thailand’s longest tunnel – at 1,362m. Khun Tan Station, on the north end of the tunnel, is also the country’s highest -sitting at 758m.
Digging for the tunnel commenced in 1907. It took eleven years to complete. It probably shouldn’t have, but the digging was disrupted when the engineers were arrested. They were Germans, you see, and Thailand reluctantly declared war on the Axis Powers in July 1917, not long before the tunnel was set to be completed. It got completed by a Thai engineer in 1918.
Originally the tunnel was meant for a standard guage train, but it was later switched to a metre-guage, leaving extra space inside the tunnel.
Incidentally, can someone tell me what weird creatures are decorating the north side of the tunnel? I went past it at night and couldn’t quite make it out. I read somewhere that the ashes of the architect of the tunnel – Emil Eisenhofer – are located in a small shrine somewhere in or on the tunnel.