Better yet, scrap the network of cameras and employ a load of uniformed, unarmed police officers wearing body cams. (The cameras should default to deleting footage after a few hours, unless the officer presses a button to save it, and all arrests not recorded due to "missing" footage are deemed invalid).

This helps communities to feel the reassurance of a trusted police presence, creates local jobs, and provides a decentralised alternative to having a single network controlled by a few hidden, unaccountable individuals. Putting a human conscience behind every single camera seems like a good way to prevent tyranny and encourage whistle-blowers.

Yep, just shoulder surf the passcode the user enters on the train, or read their fingerprints/retinal patterns off their body and print it out onto your key material.