Crusade against Wikipedia that the information is stupendously risky if you think that's a good use of your time.
https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=OJ:L...
(p. L 342/109-110, or ^F RADIOSTOP)
This is Official Journal, available in all EU languages (at the time).
Also the risk is .... not existent. To damage railway infrastructure there are way more efficient and simple ways (getting a rail to derail, destroy some signaling cables, ...)
The "attack" here would require quite some investment in equipment (power etc.) and only bring a single train to a realtively quick stop, well in safe margins, where the driver after a quick check with the dispatcher can override and continue the ride, while investigators would collect the equipment.
It's impractical for a stunt (way simpler and more effective: just walk close to the tracks and they will do emergency stops for all trains close by) and ineffective for an attack.
For logner tunnels etc there is an override, where emergency break doesn't break immediately as stopping a train mid-tunnel is most of the time the worst choice. Smoke in tunnel is dangerous. Emergency help can't reach it. Evacuation has confined space. etc. There emergency break rings an alarm with the engineer. If they ignore it, it will stop after a while, but usually they will use intercomm to understand the reason.
A way however you can stop trains on a German train line is by putting a 2kHz electromagnet on the tracks. That will tell the train that it just crossed a red signal and will stop.