>There’s another distance limit at work here, and that is the speed of light. It takes milliseconds for the signal in your phone to reach the hotel above ground and be handed over to the mobile network. But if it takes too long to get from phone to hotel, then your phone call s..a.rt..s..t o. br..e..ak up. As it happens, that distance is about 12km, so Boldyn needs nine hotels around London to cover the whole of the Underground

I find that interesting. Another fascinating rabbit hole the article has sent me down is that there is an unused station called north end. I've been down that stretch before and i had no idea. Does anyone know if passengers can see it?