Transport for London are currently trialling in-tunnel 4G service on the Waterloo & City line, with a full rollout across the whole London Underground network p

Transport for London are currently trialling in-tunnel 4G service on the Waterloo & City line, with a full rollout across the whole London Underground network planned for 2019. https://tfl.gov.uk/info-for/media/press-releases/2017/november/4g-mobile-coverage-on-tube-on-track-to-begin-from-2019 https://tfl.gov.uk/info-for/media/press-releases/2017/novemb... The tunnel gauge isn't a major problem. The standard technology for providing wireless coverage inside tunnels is a leaky feeder or radiating cable - a length of coax cable with a perforated shield. The leaky feeder acts like an antenna with a very elongated radiation pattern. As long as the distance between stations isn't too great, then you don't actually need to put any equipment inside the tunnel other than a length of cable. https://en.wikipedia.org/wiki/Leaky_feeder https://en.wikipedia.org/wiki/Leaky_feeder