I've noticed many metro systems in cities do somehow receive GPS and cellular networks which I'm always impressed by. London on the other hand, has only recently started receiving signal for cellular networks, and rollout is slow.

GPS does not work at all. I've always thought using the WiFi access points that have been installed on the underground could be a great addition to something like Citymapper to figure out where you are located even when there is no GPS.

Both Apple (WPS?) and Android (Location Accuracy) support improving location through WiFi access points and cellular network discovery. That's usually why you are able to get a lock onto your position even while underground.

Wi-Fi and Cellular is why your phone knows where it is, that already works.

I believe Google Maps on iOS can tell you where you are on the Underground. But I presume it’s not using GPS, as you say.

Phones already use WIFI for location.