"Man at B&Q" wrote in message
...
On Jan 2, 10:27 am, John Williamson
wrote:
dennis@home wrote:

"Tony Bryer" wrote in message
.. .
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

I think the biggest problem would be air flow caused by passing trains..
you would probably need to build a central wall to control it. This
would also make it much stronger.

Double track rail tunnels on the main line network don't bother, even on
high speed lines. Underground networks often use the passage of the
trains as pistons to help the ventilation in the network.

When most of the main line tunnels were built, the main problem with
ventilation was getting enough airflow to get rid of the smoke and
steam, which is why you can follow the line of most of them by walking
between the vents. In steam days, you would see columns of smoke rising
from the vents, and could watch the progress of a train through the
tunnel.

Not just the smoke in extreme cases
http://baldbrummy.typepad.com/amateu...fire-stub.html
A testament to the original over-engineering that the tunnel survived.


The ventilation shaft added to the fire acting as a chimney. Having only
electric locos and no vent shafts means the tunnel could have been sealed
from the air at both ends using JCBs. Then the fire would be extinguished.