PZB

PZB or Indusi is an intermittent cab signalling system and train protection system used in Germany, Austria, Slovenia, Croatia, Romania, Israel and on one line in Canada.

Developed in Germany the historic short name Indusi was derived from German Induktive Zugsicherung ("inductive train protection"). Later generations of the system were named PZB highlighting that the PZB/Indusi system is a family of intermittent train control systems in comparison with the continuous train control systems including the German LZB (short from German Linienzugbeeinflussung, literally "linear train influencing") that was introduced at the time. The term PZB is short from German Punktförmige Zugbeeinflussung, literally "punctiform train influencing", translated as "intermittent train protection" or officially "intermittent automatic train running control"

Originally Indusi provided warnings and enforced braking only if the warning was not acknowledged (similar to traditional automatic train stop). The later PZB systems provide more enforcement relying on a train computer.

Locomotives and multiple unit cars with operating cabs are equipped with onboard transmitter coils with the superimposed frequencies 500 Hz, 1000 Hz and 2000 Hz. Passive tuned inductors (RLC circuits) are situated at appropriate trackside locations; each inductor resonates at one of the three frequencies, depending on its location. When the leading end of the train passes over one of the trackside inductors, the inductor's presence is detected by the onboard equipment through a change in magnetic flux. This activates the appropriate onboard circuit and triggers whatever action is required based on the location (e.g., an audible/visual warning, enforced speed limit, or enforced stop).

The three frequencies have different meanings to the train:

The original Indusi protocol was placing a 2000 Hz inductor at every visual main signal that could show a red signal for an immediate stop. If the train driver overruns the red signal then an emergency stop is enforced unconditionally. The 1000 Hz inductor is a conditional restriction that is commonly placed at every distant signal that could show a yellow signal pointing to a following red signal - in the original Indusi protocol the train driver has to acknowledge the bell ring within 4 seconds or the train will be halted automatically. Based on the yellow signal the train driver is required to lower the speed to allow the overlap after the stop signal to be enough to halt the train safely. An Indusi system with a speed limiter (at least since I60R) would enforce a maximum speed after a given time in that situation with the maximum speed depending on the type of train. The 500 Hz is commonly found near railway stations or shortly before a main signal - it activates a lower speed limit than the 1000 Hz inductor. Since the visual signals may switch off while the train is moving, i.e. no red signal anymore after crossing a yellow signal, the train driver can release the train from the enforced speed restrictions using a button allowing to accelerate to the free section ahead.

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