TBM Cost per mile?

[WatcherZero]

They do serve a slightly more complicated process than just being an entrance arch. They have to handle the shockwave of the compressed air caused by the train moving through the tunnel similar to the banding around the end of a bronze cannon barrel. And in high speed railways they also have to dissipate that energy more gradually for example by having a design similar to a muzzle brake.

 

[Alfie1014]

Different geology, different station sizes, different systems, different politics.

Isn’t Manhattan built on bedrock? (mind you my scientific evidence for this is the Thunderbirds episode where they move the Empire State Building)!

 

[InOban]

If is indeed. schist I think.

 

[DJ_K666]

A portal is just the entry/exit from the tunnel, so even a hypothetical metre long tunnel would need two of them (unlikely as a 1m long tunnel is to exist!)

That's a footbridge isn't it? A very heavily engineered footbridge, mind.