Northern Line TBTC Track Loops.

One of the requirements of the PPP Contracts was that any technology used should be proven in service. When Thales was chosen around 2003 by Tube Lines to resignal the Jubilee, Northern and Piccadilly lines their system was at the time already well proven for loops but not for radio. Hence the 'safe" decision. There's no particular reliability issue although of course the loop cable, which was supposed to be highly resistant to damage, has shown in use to be more susceptible to damage from track walks, replacement rails being dropped on it and lost traction shoes shearing the wires. However, broken loop cable does seem to be almost eliminated now though.