[ratio53]

I wonder how they managed to tell trains to stop.

[lxgr]

Stop signals. For legacy train control systems, these still work visually and via wires. ETCS (starting with Level 2) does use GSM-R, but everything is fail-safe: No active communication, no movement authority, so the "virtual signal" display in the cab will pretty quickly also show "stop".

[Glawen]

Deutsche Bahn trains stop themselves all the time, no need to tell them

[sc11]

Signalling still works, so you can let the trains continue to a safe place like a station and then not let them leave until the radio issue is resolved

[PLenz]

That depends, cab signaling for example needs radio to work

[LargoLasskhyfv]

Not necessarily, at least for some time:

https://de.wikipedia.org/wiki/Linienförmige_Zugbeeinflussung