This information is derived from open signalling data, which the signalling system _has_ to track to operate trains safely and identify their locations.

The reality is the illusion of pinpoint accuracy is a figment of this site's imagination; this site is using some creative license to guess the precise location of trains.

How is it surveillance for an organization that operates a fleet of trains, to know where the trains are?

Even if there were no such thing as GPS, simply the act of running a fleet of trains or trucks or buses on a schedule, means that anyone could do a bit of math and roughly calculate the position of every vehicle based solely on the schedule.

How? Knowing the location of trains is pretty important.