On Mon, 02 Jan 2012 04:22:31 -0000, Tony Bryer
wrote:

On Sun, 01 Jan 2012 18:43:53 -0000 Polygonum wrote :
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.

Wouldn't one larger tunnel have safety issues - thinking of the Channel
Tunnel where you escape from the running tunnels into the service tunnel

Indeed it would. But I started out questioning the assertion that one
larger tunnel is cheaper to bore than two small. Your point adds to the
rationale. Though if there needs to be a parallel safety tunnel, maybe
that could be the same whether for one large tunnel or both both small
tunnels.

--
Rod