Its return train is currently stuck at Oberhausen.

It could be chaos on Dutch rail too the coming day. Trimmed down schedule on some of the busiest routes (due to the current heatwave). Add to that a labor union strike on Wednesday June 24 (that's today local time as I write this).

Although NL rail travelers are usually quite good at anticipating, and adjust their schedule. So might just be a quieter-than-usual day.

Mainly Meta Services, which seem to spike, isn't it? And Google Fiber

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".

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

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

It is telling that I thought “that’s why all trains were late this afternoon” before I realized that the issue occurred only minutes ago.