Any such failures are potentially massively fatal, and grinding to a halt is exactly what we want the trains to do. All that means is that you design the syste

Any such failures are potentially massively fatal, and grinding to a halt is exactly what we want the trains to do. All that means is that you design the system very robustly. And, of course, trains fail-to-safe. Cut a brake line? Train can't release its brakes. Signal out? Conductor can only move in GoA0 (manual), at reduced speeds, until the next signal is reached (and readable). Etc.