from the official railway administration: https://mapy.spravazeleznic.cz/vlaky

they do?

Not affiliated with the site, but it's almost certainly just the Train Describer feed from Network Rail to observe headcodes stepping through berths, and then doing some proprietary interpolation within berths to guess where the trains are, probably using fairly coarse inputs like train class, rolling stock type and line speed. It's possible it's not even doing that, and they have just built a model that observes typical passage times between berths and they average that out.

TRUST (reporting system that describes train movements as they've happened) does accept updates by means other than the Train Describer, which might include GPS if the unit is equipped, and in many areas is dependent on signallers making manual reports (which may not be made immediately). They might also use TRUST as a data source, especially in non-Track Circuit Block areas. The rate of GPS updates is not going to be anywhere near as frequent as those train position indicators that appear to move here in "real time" across the map, so however they combine data sources, their site is trying to be clever in guessing train location.

It goes without saying: this sort of map is highly likely to be wildly inaccurate and isn't useful for anything safety-critical.

I'm also close enough to a railway line to see the trains. There are two lines near us - a fast line (that I can see) and a slow line on the other side of the estate. The map showed a train moving (quickly) on the slow line, at just about the time I saw it fly past out my window. So it isn't even accurate as to which line the train is on.

Possibly useful as a funky station arrival board, but overlaying that data on an actual map is a bit misleading.

There's no way its input data is anywhere near adequate for the rate the position indicators refresh, so it's taking some base data and then just... guessing the rest.

>it's a fun toy, but I wouldn't depend on it for anything important.

This could be said for the rail network as a whole.

Neglect and underinvestment over the last 60+ years has left it in a sorry state, and debacles such as HS2 show how government has no ability to deliver proper material upgrades to the ageing infrastructure and service. The direction of travel (scuse the pun) has been clear since the Beeching cuts: roads are the priority. Add to that Neoliberal divestment policies and we end up where we are today: overcrowded, filthy, ugly trains barely fit for cattle transport and chronically understaffed stations and train crews. Not to mention the extortionate prices for a ticket to travel on the network.

I adore rail travel, but dread the necessity of using it any time I go on a journey.