Cut and cover is still the cheapest way to build subways, but is less often used nowadays because of the surface disruption.
Long before tunnel boring machines existed we needed to develop methods to dig under rivers. Brunel invented the tunnelling shield for digging under the Thames in 1825 and later a more refined version was used to dig the first deep-level tube line which opened in 1890.
Long before tunnel boring machines existed we needed to develop methods to dig under rivers. Brunel invented the tunnelling shield for digging under the Thames in 1825 and later a more refined version was used to dig the first deep-level tube line which opened in 1890.
https://en.wikipedia.org/wiki/Tunnelling_shield