Hi railway lovers. Welcome to Railways explained. Today’s topic is rail signaling and its evolution through different periods of time since the beginnings of railways. As you know railways have a complex system. It includes several technologies working together with aim to fulfill demands in terms of capacity speed and mobility. So, the railways can be divided into different systems and subsystems depending on their functionality. One of those systems is of course, the signaling system. This video is an attempt to explain in a simple way how did railway signaling evolved through different periods of development of railways and how it really works, so everybody can understand its basic concept. Before we start with historical overview of the development of the signaling system, we must first mention one basic operational rule on railways. Due to safety reasons on each segment of rail infrastructure, there can be present only one train at the same time. At the very beginning only one train could be present on the entire railway line, which was, by the way, relatively short and mostly connecting only point A with point B. However, as demand for rail traffic grew and therefore the need for more than one train on the line, it was necessary to develop operational rules and systems. That would make that possible. However, the basic premise has remained the same both for signaling of the 19th century as well as for signaling of the 21st and that is Trains cannot collide only if they are not permitted to occupy the same section of line at the same time. …When peoples were signals It was necessary to find a way for organizing operation of more than one train on the same railway line. For that purpose, railway lines were divided into sections known as blocks. In the early days of railways, men, originally called the 'policemen', were employed to stand at certain intervals, forming some kind of a "human blocks" They were placed along the line with a stopwatch and were using hand signals to inform the train driver of approaching train that preceding train had passed more or less than a certain amount of time earlier. The red yellow and green flags were used by policemen to show train drivers how to proceed. A red flag was shown in case when the preceding train has passed less than 5 minutes before the approaching train arrived. if the approaching train arrived more than 5 and less than 10 minutes after the preceding one has passed a yellow caution signal was shown to the train driver. The full speed green signal was shown only in case if 10 or more minutes elapsed since the preceding train has passed. The system was called "time interval system". The time interval system while trying to use a headway to protect trains actually created some serious problems. The policeman had no way of knowing whether a preceding train had cleared the line ahead, or simply put, he had completely left next interval, so in case of a preceding train being stopped for any reason, the crew of an approaching train would have no such information unless it was clearly visible from their position. As a result, accidents were common in those early days because, in some cases, train drivers believed they had a 10 minutes gap ahead of them, but in fact that was not the case. The reason why this is so important, is the fact that trains requires significant time and the distance in order to stop, counting since the moment the braking is being applied. Today these distances are usually no less than 700 meters or 2296 feet but they can also be 1500 meters or 4921 feet and even more, depending on different factors. Another serious problem, from the market point of view, was line capacity. Even if engineers could rely on trains not making unplanned stops, the 10 minutes time interval significantly restricted the number of trains which could run on a given railway line over the certain amount of time. As there was a need for more capacity, which meant more trains, engineers gradually begun to reduce the time or the headway between the trains. As expected ,as they reduced the headway, the number of trains per hour increased. Sounds simple, right? Well, in fact, it's not. As the capacity increased, the number of accidents also increased. Eventually, this showed the need to urgently change something. The solution was, fixed track site signaling. By the middle of the 19th century, many railway companies had developed some form of fixed track signals which aim to replace hand signals and increase line capacity. With the invention of telegraph and later the telephone, It became possible for the railway staff in charge for specific "block" to send a message to the staff in charge for the next one, confirming that a preceding train had left their block, clearing the way for the next. Mechanical signals first appeared in the UK in 1841 and the signal box with levers controlling remote signals and points in 1860. Originally, the run of each train through a section was tracked visually by the signalman. When the train had cleared his section, the signalman told the signal box operator on the approach side That his section was now clear and that he could, if required "accept" another train. The messages between signal boxes were transmitted by system of bell codes. Also, to give an early warning of signal set danger, after some time, all signals were classified as either "home" or "distant" signals. Why did this happen? Well, distant signals provided an opportunity of showing the indication or the condition of the signal ahead, I.e. whether the signal is showing "stop" or proceed. The introduction of distance signals provided an early sighting point and thus Increased the available braking distance for a train crew, thus allowing higher train speeds. "ith introduction of warnings, it also became necessary to differentiate between indications show to train drivers, and so yellow became distant signal color while the red and the green became "stop" and "proceed". In some countries, and on some railway lines, this system of mechanical signals is still in use even today, however, the next step of technological evolution broad improvement in terms of introduction of the color light signals. Today, on most modern railways, the mechanical signals have largely been replaced by the color light signals. Compared to mechanical ones, color light signals have the advantage of displaying the same aspects by day and by night, and they also require less maintenance. Although typology of signals widely vary between different countries, and sometimes even between different railway systems within the same one, a typical basic system of aspects would be the following: Green: Proceed at line speed. Expect to find next signal displaying green or yellow Yellow: Prepare for the next signal displaying red. and red stop. The basic idea was to use these signals, to again, divide railway line into different blocks To be more precise in modern signaling. There are actually two different concepts of blocks. Automatic block and the moving block. Under automatic block signaling, signals indicate whether or not a train may enter a block based on automatic, on rails, train detection indicating whether a block is clear and therefore able to accept the approaching train. As suggested, at the beginning the line equipped with automatic blocks so called interlocking system is divided into sections of length not shorter than the breaking distance of the fastest strain running on the root. If we go further into details, there are two ways of detection of occupation of blocks. The first one is using the track circuits. It works on the following way: First the rails at both ends of each section are electrically isolated from the next and previous section. Then, an electrical current is fed to both running rails at one end, and, the relay which is located at the other is connected to both rails. When the section is unoccupied, the relay coil is energized and it completes the electrical circuit of that block. However, when a train enters the section, short circuits the current in rails, and the relay becomes instantly de-energized. This simple system is then used as a basis for displaying different aspects of block signals. Track circuits can be also used to automatically detect some kind of defect, such as broken rail. The second way is axle counters. This method of determining the occupation of a block uses devices located at its beginnings and ends, which counts the number of axles that enter and leave the block. If the number of axles leaving the block equals those that entered it, the block is assumed to be clear. On the basis of this, interlocking concept, the upgraded rail signaling systems were introduced in order to increase railway safety. These new systems, able to constantly monitor the speed of the train, are called ATP or Automatic Train Protection systems. At first, ATP systems used a target speed indication and audible warnings to advise the train driver if the train, for example, passed a red signal or exceeded a speed restriction. In these cases, the system applies an automatic brake if the driver fails to respond to the warnings. One of the key principles of ATP systems is the braking model concept, a mathematical model which allows predicting the maximum safe speed of the vehicle. Once the braking pattern is known, it’s easy to determine, in real time, what is the maximum speed at which the vehicle can travel, so it can safely stop before the target or danger point. When speaking about determining the maximum speed in real time, another concept of blocks, as mentioned, is a moving block system. For moving block system, a computer calculates a "safe zone" around each moving train. The system depends on knowledge of the precise location and speed of each train, which is determined by a combination of several sensors: active and passive markers along the tracks and trainborne speedometers. With a moving block setup, trackside signals are no longer necessary, and the instructions from the tracks are directly transferred to the trains. This has the advantage of increasing track capacity to the maximum theoretical limit, By allowing trains to run closely to each other, at exact breaking distances, while maintaining the required safety margins. There is one more important aspect to mention in order to give you a full perspective of the modern signaling systems. Namely, with the progress of technology and the increase of speeds, train drivers are no longer even able to clearly recognize the signal signs provided by the trackside signals. This made the systems without trackside signals practically a must. In Europe, the speed limit after which the trackside signals are no longer used, is 160 km/h (100 mph). We will also mention that modern signaling systems are based on communication using radio technology, while the project of introducing 4G LTE technology has already been launched in South Korea. Tell us in the comments bellow if you are interested in a special video, explaining these types of technologies in more detail. As we saw, in a two-centuries long history of railways, the railway signaling went through the process of constant evolution and adaptation to new technological and market circumstances. Although, the basic principle of occupation of the segment of railway infrastructure remained the same, as demand for rail traffic grew, this segment was becoming shorter and shorter, all the way to its minimal extent possible. This actually enabled maximum theoretical capacity. On the other hand, as technology made progress, so did the way of communicating the signal signs to the train drivers: from flags, through mechanical and light signals, to the point where signals are no longer even requred and whole communication is done between the railway infrastructure and the train drivers cab in real time, optimizing the train speed and braking paterns authomaticaly. Unfortunately, we came to an end of this video. Thank you very much for your attention, we hope you enjoyed and learned something new about the railways of the world. For the end, of course, for more railway stories, don’t forget to like this video and subscribe to our chanell! Goodbye.
