In the early days of railways, trains were few and slow, and electricity was not yet used for communications, so the operations were mostly built on the principle of audible and visible signals, and on the principle that trains used to run in specified intervals.
In Great Britain, right from the very beginning, railway lines were built to intersect other roads and paths as little as possible, while in Germany, level crossings were frequent. In 1870, level crossings in Germany at average were 625 metres apart, in Austria-Hungary 833 metres, and in England 4000 metres. So in Germany, there were frequent guard-posts which could also be used to transmit signals by optical telegraphs or other visual signals.
Some railways in the Länderbahn era had the rule that the posts had to be built so that each guard could see the adjacent guards. Messages were transmitted using a variety of devices, like baskets and balls being raised as well as semaphores and tables. Transmitting messages by optical semaphores became state of the art; in 1866 the Verein Deutscher Eisenbahnverwaltungen (Association of German Railway Administrations) recommended using semaphores for all instances of fixed signals except point signals, although until 1889 the Großherzoglich Badische Eisenbahnen (Grand-Ducal Baden Railways)'s rule book contained provisions for basket signals.
When a train left the station, the station guard raised the semaphore arm. Upon seeing this, the next guard would raise his semaphore's arm if the section under his authority was clear, and so on until the next station. So, when the driver approached a semaphore with arm raised, he could be sure that it was safe to proceed. Obviously, more than one signal aspect needed to be transmitted, such as: train coming from left / right, train coming from left / right on the wrong track, train is cancelled, an assisting loco is required from left / right, the assisting loco shall return, etc.. Every railway administration used their own signals, so in 1867 there were 166 different forms for these eight signals.
Note: German railways use right-hand running, so the expression wrong track means the counter track which usually is the left one.
The trains could be used to transmit messages to the guards, too, by using differently coloured lanterns or flags for messages such as: an extra train follows, an extra train comes in the counter direction, train is running on the wrong track, train is returning, order to revise the track.
We can see that messages need to be transmitted to two very different recipients and for very different purposes:
For the former messages, fixed signals like semaphores or boards became commonplace, while for the latter ones, telegraphs and bells came into use.
As my web site focuses on the lineside signals, I will not cover the bell signals and telegraphs here.
After some development, most of railway administration introduced semaphore signals to indicate whether a train may proceed or must stop. In 1856, in the precursor of all railways, the British railways, 70% of all fixed signals were semaphores. In Germany, we call these signals Flügelsignale ("wing signals"), for they have arms which can be turned into various positions.
From the beginning there were two different philosophies of how to use signals. In the UK, the default state of a line or station was closed (i.e. a train must not enter), and a permission to enter must be given by a visible signal. In other words, British practice was that a permission to proceed is given by a positive signal.
France used the opposite approach: A station or line was open, unless some circumstances prevented a train from entering (e.g. station tracks already occupied). The prohibition to enter was displayed by a large red disk (see French disque signal) that was flipped edge-on (and thus not visible) when the line was clear, so a permission to proceed was indicated by the absence of a signal. We call this a negative signal.
Germany followed the British approach, with the German Vorsignal (distant signal) being some sort of hybrid. Apart from it being coloured yellow, it works like the French disque - when visible, it means "expect stop", when flipped edge-on, it means "expect clear". However, German railways followed the British railways thinking that the absence of a signal should never indicate a proceed aspect, so the German distant signal was amended with a distant signal post plate, a board that is always visible and indicates the position of the distant signal in case the disk is flipped aside.
Even though some countries (like France, Belgium, Netherlands, Italy follow more closely the British model while Scandinavia, Poland, the Balkan countries and Russia follow more closely the German model, there are only few railways which used signals other than semaphores. One example is France, which used a square board (carré) to indicate an absolute stop. Other differences are whether the semaphore arm is raised or lowered to indicate to the train driver that he may proceed, but all in railways, a horizontal arm means stop.
In the Länderbahn era (Epoch I), the semaphore signals in the different German states already looked similar, but they were not yet the same.
|Länderbahn signals were already similar in appearance, but due to different suppliers and standards there were still differences.|
A further development was triggered by several factors:
The German railways did the first test installation of colour light signals in 1928 on the line between Hirschberg and Königszelt in Silesia (the area now belongs to Poland, and the cities are named Jelenia Góra and Jaworzyna Śląska). The tests were very successful, since 1959, no new semaphores have been installed, though some may still be in operation. Position light signals as they are used by some US American railways have never been used in Germany (except for some shunting signals like Ra 6...9)
Another important aspect were the colours which were used to indicate the signal aspect at night (or later, as the electric colour-light signals displayed the night aspects of the semaphore signals, this concerned the aspects of the colour light signals, too).
When colours were used at first, it seemed natural to use an unfiltered light (i.e. white) to indicate an uninhibited aspect (i.e., clear), and to use a light filtered by a tinted glass to indicate aspects like caution, slow, or stop.
The first signal lanterns were illuminated by petroleum lamps which emitted a yellowish light, so yellow and white would not have been easy to distinguish, and anyway glassmakers were not able to make suitable yellow glass, but red and green were possible.
So the first colours and their meaning was:
It did not take long to discover that this choice of colours was not optimal. Some serious accidents happened when a red glass broke and consequently the approaching driver saw a white light and passed a signal at danger.
It was therefore decided that white should not be used at all at main signals. Several railway administrations experimented with new colours, mostly various shades of yellow. As said above, a main problem was that a petroleum or gas light is naturally yellowish, so a colour had to be found that is sufficiently distinct from white. Various shades of yellow and violet were tested, and eventually railway administrations opted for a rather orange shade. A reddish orange or amber has the advantage that, if confused due to poor visibility, it would be confused with the more restrictive colour red, and hence the driver would err towards the safe side. In Germany, yellow was officially recognized as third signal colour in 1910. In Epoch I, i.e. before 1919 and the nationalization of the Länderbahnen into the Deutsche Reichsbahn, the German states had the authority to make their own rulings, and so was not until 1921, that white was phased out as signal colour.
The last change in signalling colours happened just after WW II: Until that time, one green light at a main signal meant clear, while two green lights meant slow. In 1948, the lower light was changed to amber, so that from then on the "slow" aspect is indicated by a greed light above an amber one.
There is a separate page on the History of Aspects in the section of Historical and Test Signals.
A final note on colours: White is still used, but only as colour for signals such as shunting, point or subsidiary signals, never to announce or give a proceed aspect to train movements.
Beginning in 1949, some new designs of colour light signals were proposed. When the distance between two main signals becomes closer than about 1300 m, the distant signal for the main signal in advance is mounted at the post of the main signal in rear. To display a two-block aspect, i.e. the aspect for the block following a main signal in combination with the distant signal for the next block needs three or four lights.
There were several incentives to change to a simpler system of aspects, some technical so as to lower costs and make the system simpler by using less lights, and as East Germany became a member of the Warsaw Pact, the East German Deutsche Reichsbahn became a member of the East Bloc's railway organization OSShD, and one of their incentives was to unify the rules (and signal aspects) throughout the socialist countries. As a result, signal aspects between and including East Germany and Vietnam are the same or at least very similar.
The first try in West Germany was to get rid of distant signals where not needed but to introduce multiple-block signals that would show the status of up to the next three blocks by using just two lights and two different flashing intervals. This system was developed but never put into use.
The next test began in 1951 in the Köln (Cologne) area, using two lights (three for some aspects) and flashing, albeit only one interval rather than slow or fast flashes.
East Germany introduced a version of multiple block signals in 1955 on the Berliner Außenring (Berlin Outer Ring, a newly built ring railway circumventing West Berlin). These signals also used two or three lights and flashing, but the aspects were much clearer and easier to comprehend than their West German counterparts. In 1962, they were replaced by the new signals which were (with certain national variants) introduced throughout the OSShD.
West Germany tested a new version of Hp signals which were not multiple-block signals but simplified Hp signals, using flashing green rather than green-yellow for slow, and only one green light at the distant signal. However, this was tested only on one line and never introduced elsewhere.
I have a special page on historical and test signals, where I describe all of these systems:
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