In the UK, they provide Wifi in stations on the London underground, - if you’re travelling with WIFI enabled, they’ll track your MAC address through stations. (assuming you connect.)
Even if you don’t connect the infrastructure is deployed to follow probe requests from your Wi-Fi through the transport network.
(Officially the use for this technology is to track station occupancy, (e.g. give advanced warning of significant crowds, (which become dangerous) in stations.)
If you plan to break the law, it is perhaps not best to take a literal radio beacon with you!
(obviously…)