For those who don't know, every train in Poland is equipped with an automated breaking system that can be triggered over the radio. It's an, let's say "area-wide" system i.e. if you broadcast a radiostop signal it will stop all trains around you as far as your radio broadcast is heard. There is no selectivity - you don't stop a specific train, you stop all of them, it's like shouting "everybody on the ground". Also, the signal is not encrypted, the specification for it is public knowledge, and it is extremely cheap and easy to do it yourself - you can pretty much record the signal on your mobile phone - it's just 3 different tones, and later play it while holding transmit button on your cheap $20 baofeng. Because of that, there are around 600 cases of unauthorized use of radiostop in Poland every year! Imagine what it does to train schedules.

I have no idea how things like thise work in other countries, as far as I know this is not a universal solution and it looks like something that was specifically invented for Polish railroads that did not have a signalling system that would be able to properly detect and warn about two trains on the same track. I guess that other countries just have more advanced signalling than Poland... correct me if I'm wrong.

BTW, this system started to be implemented in 1987. It was probably fine at that time but nowadays anyone can buy a cheap radio, trigger it and run. The whole thing suddenly got media coverage because this time someone not only transmitted radiostop signal but also broadcasted Russian anthem and Putin's speech on a frequency used by Polish railroads (BTW they also use analog and unencrypted modes so similarly to radiostop feature, anyone can listen, transmit and jam it). There is an EU-wide railroad radio telecommunication system being implented right now (GSM-R). The problem is it has already been like 15 years since they started building it in Poland and it is still not finished, 3 years behind the schedule.

If you want to know more this Polish article has a pretty detailed info on radiostop.