On 17/12/2012 23:15, John Williamson wrote:
polygonum wrote:
I appreciate that - there are some excellent pictures of the process.
But harry seemed to be suggesting that the tunnels *were* quite
shallow - but does that mean they no longer are?

The early ones were shallow, as that was all they could build and
operate using steam traction. Then in 1890 or thereabouts they started
using the shield method of building the tunnels, and depth became almost
irrelevant except for access problems. The needed prior invention to
make these lines possible was reliable electric traction, with the coal
burning taking place in a power station and reliable transmission of
power. See:-

http://en.wikipedia.org/wiki/Lots_Road_Power_Station

There were others, but this was the most famous. Now, the tube takes its
power from the National Grid.

Even the shallow lines with their many open air sections required the
steam engines to be fitted with apparatus to "digest" the smoke, often
by passing it through the water in the side tanks of the locomotive.
This was not very effective. Cough, choke, splutter

But did they change the shallowness of, for example, the existing Baker
Street station?

--
Rod