[redleader55]

The way this is usually done is through a radiating wire that goes on the wall from station to station. This solution requires a lot of power. The stations in London are very small, leaving very little space for a transformer that can provide said power.

Because of that they rely on directional antennas that cover sections of the tunnel - but those need to be straight, otherwise another antenna is required to cover that corner. There is also very little head-room in the tube tunnels in London. You can see how things get more and more complicated as you dig deeper into the problem.