>>> I would think they use something similar on the 'Underground'
I dont think so, I'm a bit too tired now to start looking, but I wouldnt have thought that Femtocell would work in that environment.
AIUI Femtocell relies on a internet connection and a sufficiently good internet connection to carry the data over. It works ok in say a business environment that can afford to give some of their internet bandwidth to this technology. Its not really designed as an all and sundry open network for more than x number of persons.
Off the top of my head I would have thought it would be easier to put in a base station hooked directly up to their network... or relay for UG stations.