  The public transports provide an ideal means to enable contagious diseases
transmission. This paper introduces a novel idea to detect co-location of
people in such environment using just the ubiquitous geomagnetic field sensor
on the smart phone. Essentially, given that all passengers must share the same
journey between at least two consecutive stations, we have a long window to
match the user trajectory. Our idea was assessed over a painstakingly survey of
over 150 kilometres of travelling distance, covering different parts of London,
using the overground trains, the underground tubes and the buses.
