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 of German Punktförmige Zugbeeinflussung ("intermittent automatic train running control") as PZB/Indusi is a family of intermittent train control systems and it is a predecessor of the German Linienzugbeeinflussung (LZB, "continuous train protection") system. Originally Indusi provided warnings and enforced braking only if the warning was not acknowledged (similar to traditional automatic train stop) but current developments of PZB provide more enforcement.
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:
- 1000 Hz
- warning that the distant signal being passed shows "caution", drop of speed required. Driver has to confirm that he has seen the "caution" aspect by pressing a button; failure to do so within a few seconds results in a forced stop
- 500 Hz
- immediate maximum speed (Vmax) as well as further drop of speed are enforced
- 2000 Hz
1000 and 500 Hz activate conditional restrictions. If the train's driver fails to take the required action within the allowed time, the system enforces a stop. A 2000 Hz enforces an unconditional stop. Details of the restrictions vary with different versions of the system, but the common purpose is to enforce the observation of speed reductions and then a stop before a train reaches a red signal. The 2000 Hz inductor is a last resort to stop a train as it overruns a red signal.
Experiments with magnetic induction for a train protection system can be traced back as early as 1908. All of the early prototypes did require track-side electricity supply which was not available however in the widespread mechanical interlocking stations. Parallel investigations did look at optical recognition equipment (Optische Zugsicherung / OPSI) which was dumped however on the base 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 based on magnets since 1933 which is 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 is superior the German system did need to install frequency generators on the locomotive which has been a demanding endeavour at the time of steam engines being the predominant locomotive types. The Indusi system was deployed in Germany since 1934 and the system spread to Austria and countries of the historic Austro-Hungarian Empire which share a common root with Germany in terms of rail transport history during the German Customs Union.
The original Indusi system was deployed in Germany since 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 were only using 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 the war 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 on a network of 1180 km of track. 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.
Minor improvements in the 1960s led to the Indusi I 60 system. When a 1000 Hz inductor was encountered, the driver had to acknowledge the caution signal within four seconds. Additionally, a countdown was started to check whether the train had slowed to a specified speed within a specified time frame. Depending on the type of train the locomotive was hauling, the system could be manually switched between three modes of operation: freight train, low speed and high speed passenger train. In each mode, the system calculated a different speed curve based on the maximum allowable speed and braking characteristics of the train.
The original I 60 system proved insufficient in a number of situation so that it saw multiple revisions that finally led to the revised standard I 60R.
With the introduction of Linienzugbeeinflussung by Deutsche Bundesbahn the locomotives were equipped with a microprocessor-based LZB/I 80 train protection system since 1980 which was able to pick up the Indusi signals. The experience with this system led to the development of the Indusi I 60R system that required microprocessors in all locomotives. Instead of checking certain speeds at certain points in time, the new system continuously checked a curve of speed against time. If the train was faster than the curve allowed, a stop could be enforced at any time.
The PZ80 is a result of the German division and the enforcement of the inner German border since 1967. The East-German Deutsche Reichsbahn wanted to gain independence of the I 60 supply by the West-German Siemens manufacturer and later the Romanian I 60 Icret that was based on I 60. The East-German manufacturer Geräte- und Reglerwerk Teltow was advised to develop a native intermittent train control. The PZ80 supports all Indusi 60 modes enhanced with a number of new modes including speed control in steps of 10 km/h, continuous braking curves and a restrictive mode. After the German reunification in 1990 the upcoming PZB90 system was able to pick up on the experiences of the PZ80 system.
PZB90 is a new version, deployed in the mid-1990s. It features a new restrictive mode as the result of two accidents. In both cases a train had stopped at a station as intended. Then the train accelerated again, despite the signal still showing red. When the train reached the exit signal, its speed was sufficient to crash into another train despite the automatic braking enforced by the 2000 Hz inductor.
The new restrictive mode limits speeds after a train stopped before reaching a red signal. Currently, trains are limited to 45 km/h when stopping after an active 1000 Hz inductor or to 25 km/h when stopping after an active 500 Hz inductor.
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. This is a change on the old Indusi specification that had fixed intervals. The new software version can use uneven times - for example train type O must have 85 km/h after 23 seconds which had been previously specified as 95 km/h after 20 seconds. The new braking curves have been found by extensive simulation to get at a better tradeoff between security and efficiency so that train operation is optimized.
Another changes is bound to the alert functions - when a restrictive mode is extended by another 1000 Hz it does not activate the cab signal if a previous warning signal had been acknowledged. When starting from a halted position many restrictive modes could be released ("PZB frei" button) as they had been purely based on time - since version 1.6 the actual section length is controlled where the PZB restrictive mode can not be released. This led to some changes in railway stations with moving 1000 Hz inductors.
The software update of PBZ90 to version 2.0 changed some corner cases of the train control - previously it had been possible to lift any restrictive mode by changing the speed to reverse and then forward. From that version on it is only the controlled section that enforced a speed restriction. Another change is 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.
Usage by country
German EBO railway regulations requires PZB on all but very minor lines. 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. The change of allowance guidelines of the EBO did require about 800 vehicles from the former Deutsche Reichsbahn to be either retrofitted or scrapped.
An Indusi I-60 system identical to the German one is equipped on all standard-gauge railways in Romania. The Romanian rail regulator, AFER, requires all locomotives, EMUs and DMUs operating on public infrastructure to be equipped with Indusi systems.
Indusi is also installed in 13 Danish IC3 flexliner trains used on the Copenhagen - Hamburg line.
Indusi I-60 is installed on the Mecca Metro for train protection in manual(fall-back)mode.
The Indusi system has been relatively safe; however there have been two accidents that led to the creation of the PZB90 restrictive mode. One is the Rüsselsheim train disaster of 2 February 1990 - an S-Bahn rapid transit train left the station at such a speed that the automatic train stop was not able to bring the train to a halt before the next switch where another train was just crossing over. Being fully packed during rush hour the accident resulted in 17 deaths and 145 severely wounded.
There had been at least one major accident with the PZB90 in place - on 26 June 2000 an S-Bahn train left Hannover-Langenhagen station for a single-track section with an oncoming train. The PZB halted the train but the driver released the train ("Freitaste") without double-checking with the train director. 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.
The 2011 Saxony-Anhalt train collision is related to PZB in that the track was not equipped with any automatic train stop system. In the modernisation program of the mid 1990s it deemed sufficient to deploy PZB90 only on tracks rated for speeds of 100 km/h (62 mph) and beyond. 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 up to 1. December 2014.
- Ernst, Dr.-Ing. Richard (1989). Wörterbuch der Industriellen Technik (5th ed.). Wiesbaden: Oscar Brandstetter, p. 802. ISBN 3-87097-145-2.
- EBO §40. In exceptional cases, one may drive a traction vehicle without or with disturbed Indusi, but only up to a speed of 100 km/h.
- "Untersuchungsbericht - Zusammenstoß der S-Bahn 5711 mit der S-Bahn 5712 im Bahnhof Flughafen Hannover-Langenhagen am 29.06.2000 um 10:10 Uhr". 2000-09-13. Geschäftszeichen: 4012 Uub 15/00. Retrieved 2012-05-29.
- "Sechste Verordnung zur Änderung eisenbahnrechtlicher Vorschriften" [Sixth Railway Regulations Change Act]. Bundesgesetzblatt (Teil I, Jahrgang 2012, Nr. 37). 2012-08-20. p. 1703. "(the requirement is on all tracks allowing more than 80 km/h, on all tracks with multiple lines and more than 50 km/h, and on all tracks with multiple lines with any passenger line)"
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