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There are two railway tunnels in Kazakhstan and one of them is abandoned. This one is old from Russian Empire times when branch to Aulie-Ata (nowadays Taraz) was constructed.
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It is a great example of early soviet bridge architecture. At first railroad weaved its way to the bottom of Mashsat gorge and back on top. Later in 1930s overpass was constructed and it is still there.
It planned to be simple exploration adit but geologists unexpectedly opened sealed natural cavity. While being opened for everyone quality of the cave deteorated but still it is a nice place to visit if you are into digging.
A protected area, accessible on any car, road is somewhat maintained time after time. Couple kilometers in gorge there is a paleolithic cave.