Actually it's both of those, and more complex: multiple private companies building at different times in history, using widely diverse tunnelling techniques and

Actually it's both of those, and more complex: multiple private companies building at different times in history, using widely diverse tunnelling techniques and depths to suit different needs. https://en.m.wikipedia.org/wiki/History_of_the_London_Underground https://en.m.wikipedia.org/wiki/History_of_the_London_Underg...