PZB, or Indusi, is an intermittent cab signalling system and train protection system used in Germany, Austria, Slovenia, Croatia, Romania, Israel, Serbia, on two lines in Hungary, on the Tyne and Wear Metro in the UK, and formerly on the Trillium 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 (short for German Punktförmige Zugbeeinflussung, literally "punctiform train influencing", translated as "intermittent train protection" or officially "intermittent automatic train running control"), highlighting that the PZB/Indusi system is a family of intermittent train control systems, in comparison with the continuous train control systems including LZB (German Linienzugbeeinflussung, literally "linear train influencing") that were introduced at the time.

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

Experiments with magnetic induction for a train protection system can be traced back as early as 1908. All of the early prototypes required track-side electricity supply, however, which was not available in the mechanical interlocking stations widespread at the time. Parallel investigations looked at optical recognition equipment (German Optische Zugsicherung / OPSI); this was however not developed further on the basis of instability due to dirt and dust on the lenses.

Since 1931, the development concentrated on an inductive train protection system (Indusi) that did not require electricity. In a parallel development, Switzerland started to introduce the Integra-Signum system in 1933, based on similar ideas. The Swiss system did not use a resonance frequency, but a static magnetization which can only be detected as a signal when the train is moving fast enough. While the frequency induction method was considered superior, the German system needed the installation of frequency generators on the locomotive which was a demanding endeavour at the time of steam engines being the predominant locomotive type. The Indusi system was deployed in Germany in 1934, and the system spread to Austria and countries of the historic Austro-Hungarian Empire, which shared a common root with Germany in terms of rail transport history during the German Customs Union.

The original Indusi system was deployed in Germany in 1934 – it was not called by that name, however (using the full title "induktive Zugsicherung") and the shorthand "I 34" is a retrospective designation as well. The initial tests only used a train stop function (the 2000 Hz signal in later revisions) – by the end of 1934 there were already 165 locomotives equipped with the Indusi detectors and 4500 km of track were secured with inductors. At the end of WWII the system was not functional anymore and in 1944 the equipment of 870 locomotives and the Indusi signals on 6700 km of track were officially switched off.

During 1947 the Indusi resonators of the locomotives were re-enabled together with a network of 1180 km of track in western occupied zones.

The Deutsche Bundesbahn started an effort to standardize the function of a modern Indusi system leading to the Indusi I 54 specification in 1954. This included a new frequency generator that did not require three motors but only a single transistor frequency generator with a downstream audio crossover to emit the three frequencies in parallel.