1) DSD/Vigilance. (Drivers Safety Device) Usually a pedal that has to be kept depressed while the train is either in forward or reverse gear. The DSD is linked to the emergency brakes so that failure to keep it depressed while the train is in forwards or reverse will trigger a brake application.
The vigilance device is part of the DSD pedal. This sounds bleeping tone every 90 seconds if no movement of the controls are detected. The driver is then required to release and reapply the pedal in order to cancel the vigilance device and prevent an emergency brake application. On trains fitted with GSMR, a failure to cancel the DSD/Vigilance also sends an alarm to the nearest signal box/panel to alert the signaller.
This covers your illness/unconscious angle.
2) AWS/TPWS (Automatic Warning System and Train Protection & Warning System).
Both linked to the trackside signals, with the added bonus that TPWS can also prevent trains from passing signals at danger by large margins and can also be used in conjunction with permanent speed restrictions.
AWS has been around for a very long time in various forms (since the early 1900s with the GWR). It consists of fixed electromagnets in the 4 foot of the track and receiving gear on the train. A green signal activates a bell or 'bing' in the cab, turns a cab indicator to all black and requires no further action on the driver's part. A signal showing red, yellow or two yellows activates a horn, turns the cab indicator to black and yellow and requires the driver to press a button to cancel the horn. Failure to cancel the horn in time (6-8 seconds I think) triggers an emergency brake activation.
TPWS is a bit more complicated.
A train passing over the first arming loop sets off a timer in the TPWS system so that by the time the train passes over the second trigger loop the system knows how fast the train is going. If the speed that the train is travelling at is too fast to stop at the red signal/dead end platform road/reach the posted speed restriction, a 'TPWS intervention' is triggered which applies the emergency brakes. The second set of arming and trigger loops are at the signal themselves. When the signal is at red, the trigger loop is energised and should the train pass over it as it passes the signal at danger, the system will activate and make an emergency brake application.
However, there is a temporary isolation button in the cab as well as an isolation switch. These are used when the signaller instructs you to pass a signal at danger. The temporary isolation button disables the TPWS for a short period of time (30 secs for a passenger train, 60 for a freight train I think is right) before the system becomes live again. The isolation switch isolates the system but is usually in a cabinet somewhere with a seal on it. If a driver were to suddenly go 'rogue', the signaller would see the unauthorised movement on his panel and, hopefully be able to send the train into a loop, through catch points or just stop other train movements to avoid a collision.
So that answers your question about ignorning red signals.
And finally, train brakes are designed to be fail safe. To apply the brakes, air pressure is released
from the brake system, so a sudden loss of air pressure, break in the brake pipe etc would discharge all the air from the system and apply the brakes fully.
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