[samhw]

Yeah, I'm not sure non-Londoners appreciate this. The escalators at Angel (where I grew up) go 60m deep. Someone skiied down them and hit 30mph: https://en.wikipedia.org/wiki/Angel_tube_station#Escalator_s... https://youtu.be/rlF4nRUbKmc?t=53.

[DoughnutHole]

This is something I noticed experiencing undergrounds in Germany after previously only experiencing them in London - London's escalators are insane, the Cologne U-Bahn is practically surface level by comparison.

If I were to hazard a guess this is because of the soft clay that London is built on - on the one hand the soft material makes it very easy to dig tunnels, on the other hand if those tunnels are dug too shallow they will very easily destabilise the foundations of the buildings above. So you have to to build them at a depth which would be prohibitively expensive somewhere built on harder rock.

[andylynch]

Not only that, as Crossrail demonstrated one must also avoid the numerous existing tunnels (not just trains, but also utilities and even rivers). Not all of which are precisely mapped or even know about! Even then, the gap between tunnels in some stations is on the order of centimetres

[wikfwikf]

Some tube lines are very deep but there are also several which are cut-and-cover, ie at the same just-below-street depth as the NY subway.