LZB (Linienzugbeinflussung = train control system / cab signalling) is a continuous working train control system that provides full supervision for a train movement. The LZB system is designed by Siemens and was introduced in 1960/1970 in Germany on mostly the high speed lines.
If a train driver is running with 140 km/h he is going down the tracks with 38.88 meters per second. If there is 25 meters visibility he has 0.64 second to observe and react to a signal aspect outside. A trained train driver CAN do that, but it is a real challenge! Going with 160 km/h, (44.44 m/s) this observation time is 0.56 seconds. That's almost a blink of an eye.
In Germany the railway set a standard that above a speed of 160 km/h a train driver cannot humanely and reliably observe a signal aspect at such a speed.
Therefore a cab signalling system was designed that translates the outside signals to a set of indications in the train cab. The system was also designed to fully supervise the train speed and operate the train fully automatically. If a train does NOT have this full supervision cab signalling system, the maximum allowed speed of the train is 160 km/h.
Principles of LZB
The LZB system consist of a train borne computer and a lineside computer next to LZB-computer in the signal box. With a special antenna-wire build in in the track, the LZB-computer and lineside computer can communicate with the train borne computer and vice versa. This goes via a system of simple data telegrams that are constantly send to the train. These data telegrams provides speed and locator information for the train borne computer. The train borne computer calculates the current maximum allowed speed (based on the train data, braking capability, length of the train and distance to the next speed reduction), the distance to the next speed reduction and the target speed. These three critical information items: maximum allowed speed, target distance and target speed is shown to the train driver and he can choose to let the train follow this data automatically or manually operate his train in such way to stay within these given data. After 40 years working, the system has been proven to be very reliable. Only very few irregularities have taken place.
Train Picks Up LZB Cab Signal
For a train to run under cab signalling rules, the train must enter an entry point, marked by little LZB signs next to the track. The train driver may expect to be "picked up" by the cab signalling system when his train is equipped with the LZB system and it is operational and the track side system is operational and his whole train length is within the LZB area.
If this does NOT happen, the train may continue with his normal operation speed, but limited to 160 km/h.
If the train is picked up by the LZB system, the train driver will adhere to the cab signal ONLY and does not have to observe line side signals. Simply said: he must completely rely on the cab signalling and even ignore outside signal aspects. He still must be alert though and observe the line for irregularities and hazardous objects. LZB doesn't detect animals or trees on the line.
Running Under Full Supervision Cab Signalling
What Does The Signaller See?
On railway lines with LZB equipment with "whole block modus" only, the signaller can't see much actually. Like in our simulation "Zentralstellwerk Duisburg Hbf", the signaller actually can't see anything. He can't influence anything on the system, he does not know whether the train is equipped with the system or not and not even if it's operational.
And actually it doesn't really matter. The redundancy of the system is built into it. If it not works, the trains only go 160 km/h. Any break down has only a minimal effect on operations.
LZB Whole Blocks And LZB Partial Blocks
Above is a partial view of a panel of a line equipped with LZB for a high speed line. The example uses the route from signal 503 to signal 515. Between the signals are some switches and two LZB Block signs with the numbers 507 and 511. These are only simple signs outside and are only a reminder for the train drivers running under LZB. For any other driver this sign has no meaning.
In Situation A the equipment is in default state. Signals are red and the LZB blocks also show red. Before signal 503 is a LZB-Stop ("LZB-Halt") from the LZB system.
In Situation B a route is set from signal 503 to signal 515. The signal 503 shows "proceed", its distant signal shows "expect proceed". Both LZB Blocks are clear as well and at signal 503 as well as the LZB blocks have a LZB "proceed" ("LZB-Fahrt") at their location. Any train may proceed and enter the "whole block". Train 4711 will now proceed until signal 515.
In Situation C train 4711 has stopped before signal 515 and train 4712 is approaching signal 503. Train 4712 is NOT equipped with LZB cab signalling system or it is not operational. Signal 503 will show stop and the train will stop at the signal, as it is observing lineside signals. Only if train 4711 clears the whole block (as in B) train 4712 can proceed.
In situation D train 4711 has stopped before signal 515 and train 4713 is approaching signal 503. Train 4713 is equipped with LZB Cab signalling system and it is operational. The interlocking system will call the route but signal 503 will NOT show proceed. The LZB train borne system will now negotiate with the line side computer and tell the computer that the train is fully equipped. The line side computer will now trigger the signal 503 to be dark. It's not broken but turned-off by the LZB line side computer. The same computer will now put a LZB-proceed before signal 503 and the train may proceed beyond the dark signal. The signal has no meaning for the driver, as he is observing his cab signals only. The train 4713 now enters an occupied block, but is supervised by the cab signalling system. The train can proceed over the distance shown in "D" until the LZB-stop in front of LZB Sign 511.