There are only three things protecting your privacy - legislation, cost benefit, and combinatorial explosion.
Almost anything can be done with video analytics (if you have enough money and time) but :
- it has to be legal, and
- at some point the numbers get too large and the electricity required to power the computers would exhaust the resources of the earth
For example tracking every UK citizen (60m) through Waterloo station (100m passengers per year) would require 6 quadrillion comparisons. That's roughly the same number of comparisons as there are ants on the planet.
If the system was 99.99% accurate you'd have about 600 billion false matches to dig through. That's 1,644 million errors per day.
So if we all club together and everyone in the country manually checks 30 results each per day, we could implement total surveillance of a single busy train station.