"Terry Casey" wrote in message
...
In article ,
says...

On Sun, 01 Jan 2012 18:30:22 -0000, dennis@home
wrote:

Its cheaper to bore one tunnel these days.

How much would it cost to bore, says, one mile of dual tunnel capable of
carrying two tracks, and how much for one tunnel large enough for both
tracks? Make any assumption you like about the specification of the
trains
- e.g. Circle line standard.

The Circle is an Underground (sub surface) line - mainly cut and cover.

The deep tubes are bored.

You can take a tube train on the underground but you can't take an
underground train down the tube ...

That is rather pedantic. Both are tunnels no matter how they are
constructed. Cut and cover is superb, as in Paris, as the stations are just
below ground level and can be only be a few steps down. Some Paris platforms
can be seen from the street pavement. People just hop and off the quiet
trains with rubber wheels.

The first deep level underground railway was the Mersey Railway under a very
deep and wide estuary - 90 foot deep at low tide at one point. The stations
at each end are so deep, around 130 foot, that only lifts could be used -
the escalator was not in general use at the time. The stations were also
the first deep stations - not cut & cover.

Two tunnels are used for obvious safety reasons. With an accident in one
tunnel there is access to the next via fire doors for evacuation and
emergency services access - look at the Liverpool Kingsway road tunnel.
Tunnels is cheap these days -no land purchases and NIMBY court cases making
delays.