In General you can sum up the German PZB to "it got created before there were electronics around and got extended every time a major accident happened because we couldn't afford a new system for the entire network and trains." At least our signalling system is very straightforward, looking at the US or OSShD.
>>1463967The 500Hz Magnet is placed about 250m ahead of something limiting your speed to 30km/h or less. That also includes red signals and dead ends. The Moment you pass the 500Hz magnet, the red "500Hz" light turns on and the PZB device monitors your speed beginning at 65km/h, gradually lowering to 45km/h. If you are driving a freight train, those speeds are lower (read into the PZB modes O [upper], M[iddle] and U [lower]).
In case you came to a stop amid a PZB surveillance situation (passing an active 1000Hz or 500Hz magnet), the lights start to blink, indicating that you are in restrictive mode, limiting your speed to 45/25 km/h. This is ensure that in case you accelerate against a red signal (e.g. after a stop at a platform), you will come to a safe stop within a short distance. We had a pretty serious accident once where a commuter train did exactly that, passed the red signal at such a speed that the PZB-induced emergency brake stopped in on the through tracks behind the station where it got "picked up" by an intercity train scheduled to overtake. Many casualties, leading to the invention of the restrictive mode.
