Instead of relying on conventional sensors, these devices use clouds of atoms cooled to near absolute zero. At those temperatures, atoms start to behave strangely — acting as both particles and waves. As the atoms “fall” through a sensor, their wave patterns shift in response to acceleration. Using what’s effectively an ultra-precise optical ruler, the system can read these changes with extraordinary accuracy, without needing satellites at all.

Is it really too much to place a 50p zigbee proximity sensor, at 100m intervals along a line, and mesh them together?

The totality of tube line sprawl is near 400km[0]. Quick math:

• 400e3/100 = 4000 sensors needed
• 4000 * £0.50 = £2,000

There, I just saved the goverment a million quid

0: https://en.wikipedia.org/wiki/London_Underground_infrastructure#Lines