John Williamson wrote:
The Natural Philosopher wrote:
John Williamson wrote:
Tunnel sizes
Four times, as doubling the bore diameter increases the bore area by
four times. So, boring a double track tunnel costs twice as much
(roughly) per route mile as two single track tunnels. The tunnel
walls also have to be built stronger, increasing the costs further.

Not necessarily.

Single tunnels have a lot more diameter than the train, but a double
tunnel will be a lot closer to 2 trains.

I don't think that's quite right. All tunnels are made to fit the
loading gauge of the system, but single track tunnels can follow it more
closely than twin track, assuming both tunnels have a circular profile.
It's possible to make tunnels elliptical in cross section, but more
expensive in most ground. In decent rock, you can make the tunnel any
shape you like, of course, but how many cities are built on decent rock?

An advantage of a double track tunnel is that you can use the wasted
space above the loading gauge and below the track level to carry pipes
and cables.

Yes.. I realised that after looking at the picture posted earlier. Not
much ABOVE the train tho..just under track space is all.


And the tunnel biring machines make round holes. So really it is what
suits them I suppose.

By the way, cities that are built on rock will not normally use bored
tunnels. Its fiendishly expensive to go through rock. Cut and cover is
far more likely.

Viz this extract fromn the wiki entry on the NY subway

"When the IRT subway debuted in 1904, the typical tunnel construction
method was cut-and-cover. The street was torn up to dig the tunnel below
before being rebuilt from above. This method worked well for digging
soft dirt and gravel near the street surface. However, mining shields
were required for deeper sections, such as the Harlem and East River
tunnels, which uses cast-iron tubes, segments between 33rd and 42nd
streets under Park Avenue, 116th Street and 120th Street under Broadway,
and 157th Street and Fort George under Broadway and Saint Nicholas
Avenue, all of which used either rock or concrete-lined tunnels.
About 40% of the subway system runs on surface or elevated tracks,
including steel or cast iron elevated structures, concrete viaducts,
embankments, open cuts and surface routes. All of these construction
methods are completely grade-separated from road and pedestrian
crossings, and most crossings of two subway tracks are grade-separated
with flying junctions."