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Seems like these machines are kept very busy

I believe that many of them are actually single-use. Some (or perhaps all?) of the Crossrail TBMs are being buried at the end of their tunnel. Apparently the effort of removing them isn't worth their resale value?

That tunnel talk page does mention that several TBMs are being reused though so perhaps this is changing with newer/larger machines.

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They operate in an extremely hostile environment, which takes its toll, and a bit like fighter planes there aren't enough built to make a consume-quality product. Some are reused but they'll always go through a major overhaul in between doing so.

At least one of the Chunnel TBMs was also driven in to its own grave.

If you do not take them out (which would have meant dismantling them and dragging the parts through kilometers of tunnel), they have to be, or they would block the tunnel they dug.

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