I don't think any of the signatories are under any illusions as to the consequences of a lack of action by the board/senior management.
Resign from training or be sacked.
They've voiced their concern to management that the travelling public is at risk.
The next step is to voice their concerns to those who are at risk. ie, the public through the media.
The next step is resign from C and T.
You can't remain a custodian of a safety culture that you believe is unsafe and a threat to the travelling public.
It'd be like the head of Corporate Safety saying we're unsafe and no one is listening, yet stay in the job.