Here's a rare snapshot (for those of you interested in network details...) of a splitter/combiner for GSM coverage inside the Paris metro. An interesting detail: The component covers all frequency bands between 870 and 2170 MHz, i.e. both GSM bands are supported as well as UMTS. So while I don't know if UMTS capable antennas have been installed underground, at least the cabling and passive components seem to support 3G once they want to upgrade the underground system. So, how about it?