In article , Aidan Stanger
writes
The main obstacle in that area is not vaults nor gradients - it's
foundations! Many City buildings (especially the taller ones) have
foundations that go down a very long way, preventing any tube line from
getting through.

However, planning has taken this into account, with gaps being left in
the lines of buildings. One of the constraints of the route of Crossrail
is threading it through the existing gaps in the building line in the
City.

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Clive D.W. Feather, writing for himself | Home:
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