Brian Micklethwait over at Transport Blog comments on being stuck in a stationary tube train on the District Line, and that the passengers discovered to their surprise that their mobile phones worked in the tunnel.

Sadly, it cannot be deduced from this that we will in future be able to use our mobile phones on the tube. There are two ways to build an underground railway. The simplest is "cut and cover", where you dig a trench, build a railway in the trench, and then build a ceiling over the trench. The other is a deep bored tunnel, in which a tunnel is simply dug underground. The District line is a cut and cover line, and as a consequence is not very deep, and I guess the radio waves can penetrate that far. (There are technical reasons why the radio waves from O2 and Vodafone can penetrate further than those from T-Mobile and Orange, so I would be interested to know which networks the passengers were using).

Of these two ways of building a tunnel, cut and cover is obviously cheaper if the local geography is relatively flat and there is nothing of significance already built over the top of where you want to build the underground railway. In a developed city, it is sometimes possible to dig a cut and cover line directly under an existing road, if you have plenty of long and straight roads and don't mind closing or narrowing the road for a few months. (When I was last in Singapore, this was being done for the new line through Chinatown, which I found interesting as I had never seen a cut and cover railway being constructed before. Also, most of the subways in Manhattan were built with cut and cover, because their grid street system made it relatively easy). Also, it is sometimes possible to build a cut and cover railway under a park, as was done with the city circle railway in Sydney in the 1930s. However, in a developed city, new railways generally have to be built using deep bored tunnels, and this is certainly the case for any new lines to be built in London. These are generally far too deep for mobile phone signals to penetrate the ground to them.

That said, it is possible to build mobile phone base stations actually in the tunnels, and if you do this, mobile phones can then work in tube trains. (This has obvious positives for tube trains and the like). This has actually been done in Hong Kong, so mobile phones do actually work on underground trains there.

Regardless of all this, I make a point of always having a good book with me.