[dr_lew]

However a common problem causing wrecks is crews fallen asleep or that otherwise blow by a signal.

I would suppose that a crewman could spend years, or even an entire career without ever encountering an unexpected red signal, and if this is true, it would make anything approaching 100% vigilance virtually impossible, in human terms.

[HiTech RedNeck]

Well, the PTC ought to obviate situations like that, and probably will. Other malfunctions of the system could have interesting results, though.

[libstripper]

The red signal is usually preceded by yellow or flashing yellow.