Punktförmige Zugbeeinflussung

Developed in Germany, the historic short name Indusi was derived from German Induktive Zugsicherung ("inductive train protection").

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.

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.

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.

With the introduction of Linienzugbeeinflussung (LZB) by Deutsche Bundesbahn the locomotives were equipped with a microprocessor-based LZB/I 80 train protection system.

They wanted to gain independence from the technically obsolete I 60 supply by the West-German Siemens manufacturer and replacement imports of the Romanian I 60 Icret.

The software update of PZB90 to version 1.6 had important changes to the braking curves: for most train types the target speed was lowered while allowing a longer time interval.

The new braking curves have been found by extensive simulation to get a better tradeoff between security and efficiency so that train operation is optimized.

Another change was a malfunction when the train had been halted directly over an inductor that could only be released by using the fault reset which however would also drop all speed restrictions from external signaling.

The three frequencies have different meanings to the train: Warning that the distant signal being passed shows "caution", drop of speed required.

Driver has to confirm that they have seen the "caution" aspect by pressing a button; failure to do so within a few seconds results in a forced stop.

Because of the different mass and braking capability of each train this can only be asserted based on a given maximum speed that must be maintained at the point of the red signal.

The basic part of the operation scheme (German "Betriebsprogramm") of the PZB90 protocol does still use the three inductor types as seen in the following picture.

Since 1998 all traction vehicles must be equipped with Indusi in Germany – before that it was possible for trains without a protection system to use PZB-enabled lines up to a speed of 100 km/h.

An Indusi I-60 system identical to the German one is equipped on all standard-gauge railways in Romania, including the lines M1 and M3 of the Bucharest Metro.

The Romanian rail regulator, AFER, requires all locomotives, EMUs and DMUs operating on public infrastructure to be equipped with Indusi systems.

In Ottawa, Canada, OC Transpo's O-Train Trillium Line originally used German-built Bombardier Talent trains equipped with Indusi.

As part of a full signalling renewal, Siemens Mobility will equip the line and rolling stock with a new continuous Automatic Train Protection (ATP) system.

On the Metro extension to Sunderland, Indusi has been installed on the Network Rail tracks, because it does not interfere with NR's TPWS signalling system.

The investigative report notes that there had been 22 similar recorded occurrences until that time when a driver related the PZB halt to a different cause than having overrun a main signal – the report concludes that the operations manual should be changed in that double-checking with train director should not only be required on a main signal overrun but explicitly on all PZB-related stops.

This would allow some local railways to keep up with their normal operations when they had no need for their rolling stock to run on any main line.

After the accident Deutsche Bahn promised to check all single-track lines so that they are either equipped with PZB or FFB (Funkfahrbetrieb – radio-controlled operation).

The German legislature has enacted a requirement that most of the remaining minor railway tracks need to be upgraded with an automatic train stop by 1 December 2014.