The fallback for GSM-R is the normal GSM network, but according to informed guesses I've read, the handsets still need to authenticate using their GSM-R credentials (it's just normal GSM roaming), and that's failing too.
Very interesting, I always thought it was completely separate infrastructure by design. I wonder what they'll do once commercial GSM networks throughout Europe are shut down?
In the long term, sure, but https://en.wikipedia.org/wiki/FRMCS is some way away, and I doubt they'll upgrade cab radios to 5G for the fallback path alone.