Mobile phone reception in trains is invariably patchy, and certainly in tunnels you should expect to lose it. Worst is the line out of Kings Cross where in the first 20 minutes or so there are about 6 tunnels. Cuttings and lineside buildings also cause a problem. It does vary from journey to journey, seems to depend how fast your phone can keep up with changing signal strength.

Believe it or not, phone reception along the railway is not a commercial priority for the mobile phone companies. They are not measured on it as part of their franchise (that is done at static points) and there is nothing like the revenue they get from motorway traffic, where they add transmitters in order to maintain signal and thus revenue.

The railway companies realised some time ago that they could not rely on mobile phones for their own operational purposes and installed their own network whose transmitters you can see alongside the tracks. On the GNER line out of Kings Cross they have installed wireless on the trains, when it first came in there was an interesting and detailed article on their website about how they overcame all the problems they had.

The construction of the train also plays a part in reducing the signal strength, and your relative position inside the car and the extent of the signal shielding will be part of the issue. I've seen nothing about the 25,000 volt power cables affecting things, but some of these are right behind the panelling and must generate some EMF interference.

The forthcoming scheme on the Underground will give coverage inside stations but not in the trains. One is straightforward, the other not. If you want to make a call you will have to get out at a station and make it while waiting.