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Signalling block system

From Wikipedia, the free encyclopedia

A block instrument on the Midland Railway
A block instrument on the Midland Railway

Signalling block systems enable the safe and efficient operation of railways by preventing collisions between trains. The basic principle is that a route is broken up into a series of blocks, only one train may occupy a block at a time,[1] and that the blocks are sized to allow a train to stop within them. This ensures that a train always has time to stop before reaching another train on the same line. A block system is referred to as the method of working in the UK, method of operation in the US and safeworking in Australia.

In most examples, a system of signals is used to control flow between the blocks. When a train enters a block, signals at both ends change to indicate that the block is occupied, typically using red lamps or indicator flags. When a train first enters a block, the rear of the same train has not yet left the previous block, so both blocks are marked as occupied. This ensures there is slightly less than one block length on either end of the train that is marked as occupied, so any other train approaching this section will have enough room to stop in time even if the first train stops dead on the tracks. The previously occupied block will only be marked unoccupied when the end of the train has entirely left it, leaving the entire block clear.

Block systems have the disadvantage that they limit the number of trains on a particular route to something smaller than the number of blocks. Since the route has a fixed length, increasing the number of trains requires more blocks, which means the blocks are shorter, which means the trains have to operate at lower speeds in order to safely stop.[a] As a result, the number and size of blocks are strongly related to the route's overall route capacity and cannot be easily changed without changes to the signals all along the line.

Block systems are used to control trains between stations and yards, and not normally within the yards, where some other method will be used. Any block system is defined by its associated physical equipment and by the application of a relevant set of rules. Some systems involve the use of signals while others do not. Some systems are specifically designed for single track railways for which a danger exists of both head-on and rear-end collision, as opposed to double track, whose main danger is a rear-end collision.

YouTube Encyclopedic

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  • ✪ Controlling Trains - Network Rail engineering education (3 of 15)
  • ✪ Absolute Block Signalling Animation
  • ✪ indian railway automatic signal system
  • ✪ Future Railway Signaling: A Scenario for 2033
  • ✪ Make your own Signaling System (Video#13)


[train passing] ♪ pizzicato background music ♪ (Narrator) Britain's rail network transports 3 million passengers and 400,000 tonnes of freight a day. With hundreds of trains using it at any one time. All this traffic presents us with a safety challenge. Trains are guided by rails, so it's impossible for them to swerve or pull over. Trains are heavy, can't stop quickly and frequently operate at speeds which do not enable them to halt within sighting distance of the driver. Under these circumstances, one might assume that trains are prone to collision. In fact, rail is the safest mode of transport in Britain. And that's because trains are carefully controlled. Hence our responsibility at Network Rail to control them. Signalling is the control process Network Rail uses to operate trains safely, over the correct route and to the proper time-table. The two key features of this process are line-side signals and the block system. Trains can't collide if they're not permitted to occupy the same section of track at the same time. So the network is divided into sections known as "blocks". Normally, only one train is permitted in each block at any one time. The British rail network uses line-side signals to advise the driver of the status of the section of track ahead. Most line-side signals are in colour light form, but a significant number of semaphore signals remain on secondary lines. The semaphore consists of a mechanical arm that raises to signify go or lowers into the horizontal to signify stop. The most modern signals have 4 colour aspects. A green light indicates clear. A double yellow indicates that the next signal will be a caution. The yellow signal indicates caution, and that the next signal will be red. And a red means stop, otherwise known as danger. It's prohibited to pass a signal at danger. The British rail network was originally controlled by thousands of manned signal boxes located at regular intervals along the lines. ♪ guitar background music ♪ (Stewart) My name's Stewart Sentence, I'm the signaller at Uttoxeter signal box. This is the most traditional form of system on the railway as it is at the moment. A lot of this, as you see, goes back to when the original railway started. As far as we're concerned the universe begins at Caverswall over to the right and Sudbury there and we're in the middle. This set of blocks tells me where the train is between myself and Caverswall, and this set of blocks tell me where the train is between Sudbury and myself. These levers here will operate the points for the crossings into the loops and sidings. They'll also work the semaphore signals. (Narrator) To prevent a collision caused by human error, the safety system called "interlocking" protects the railway network. Interlocking is a series of mechanical devices that prevents the signaller operating appliances in an unsafe sequence. (Stewart) What you have here is what looks like a simple lever system but is actually, if you looked underneath the box, is quite a complicated interlocking system. The interlocking system prevents me giving a green signal to an approaching train unless I set that route in that interlocking system safely first. It sounds simple and it basically works simple but the action what it does is very good. (Narrator) Level frame signal boxes, while effective, aren't efficient. They only cover a short section of line and manning them with skilled operators is expensive. (Stewart) Now I can pull the signals off No. 2. [loud click] Some of these you'll see me pulling quite 'ard; that's because there's a lot of gape on these. Some people can't actually pull 'em at all. Well a lot of it's fairly hands on. You see the trains, you've got control over the trains and the job itself. It's a good job; a better job as I've ever 'ad. Without a doubt. [clank] [train passes rapidly] (Narrator) The next big leap in rail signalling control came with the electronic age and the advent of Power Signal control Boxes like this one in Derby. ♪ 60s electronic background music ♪ (Signaller) This location opened in 1969, and when it did open it represented a massive step forward to the railways in the way that trains are signalled. Well, these lines represent mainly the Derby to Birmingham main lines. This signal box actually took over 84 mechanical signal boxes, making it a far more efficient way of carrying out signalling. (Narrator) Routes are set by pressing buttons on a large control panel. Each section between buttons represents a stretch of line formerly controlled by a lever framed signal box. (Signaller) It's very easy to work around. The signalling system is very user friendly and very easy to see the layout of the trains and where they're coming from and going to. The presence of a train is indicated by these red lights on the panel. They're activated by the completion of an electrical circuit when the train's wheels pass over the track circuit. The operation of the signalling equipment is carried out by pulling and pushing the actual buttons that are set in the panel. To set a route you press the entrance button, you press the exit button and the signalling system between detects all equipment that's located between the two signals. Once that's in the correct position, the signal will clear for the train to proceed. To take the route out, we simply pull the exit button and the route will drop out. (Narrator) Power Signal Boxes are regulated by a relay room, a little like a giant mechanical computer. (Signaller) This is the interlocking room, underneath the operating floor of the Power Signal Box. And in 'ere are all the banks of relays. And these relays relay all of the information from the touches of the buttons upstairs from the signaller outside to the points and the track circuits and the level crossings. (Narrator) Relays are interlocking electro-mechanical switches. When the signaller sets a route in the upstairs control room, you can hear the switches clicking, working out how to set the signals and switches and crossings and whether the set route is safe. [clicking] (Signaller) These cabinets are where the equipment in Derby PSB reach the modern era. These allow transmission of the train head code, the four-digit running number that we saw on the panels upstairs to be transmitted to adjacent signal boxes to give them advanced notification of that train coming so that train can be routed further down the line. (Narrator) Powered Signal Boxes are effective and safe. But at Network Rail we're now introducing an even more efficient form of signalling control. ♪ rapid piano background music ♪ (Jason) Compared to the oldest lever box signal boxes, this is a world apart. It's like an Air Traffic Control Centre basically, but controlling trains instead of aeroplanes. My name's Jason Jones, I'm a signaller and I work at Ashford IECC in Kent. The IECC stands for "Integrated Electronic Control Centre". All the signalling in this signalling centre is controlled by computers. A timetable is downloaded every day and any alterations etc. are all programmed into the computer. When everything's running on-time and all the trains are in their correct place and there's nothing else going on, the computers are all running the job and I am literally just sitting here monitoring. Hello John, yeah it's sitting on area 83 Ashford, over. At any time there could be an emergency of any description and that's when I will then step in and take over from the computer. I will turn the computer off and then run the trains manually using the keyboard or the tracker-ball system that we've got. On this screen here I can see the exact layout of the stations and the tracks. I can see where the trains are - where the red line is. Each red line indicates the location of the train. I can see where the trains are heading for (what route they're taking) by the white line. That's what the computer has set up for that train to use. We can also see the signals what the driver sees out on the track. The red dots indicate a signal that's red, we've got a single yellow, we've also got a double yellow. And obviously we've got the green signals which means then he can proceed at line speed. ♪ slower piano background music ♪ The computers that Network Rail uses in this type of location are specifically designed for this type of system. They use various safety protocols, various fail-safes. You get three computers working in tandem with one another and before any decisions are made, two of the computers have to agree with one another. Ashford covers a huge area, right from the Kent coast at Folkstone right the way into Central London. That is the equivalent, yeah, of hundreds of the old style lever frame signal boxes. [train horn] We don't just deal with standard trains here. As well as the commuter trains that we run we also run the high-speed trains into St. Pancras and the Eurostar trains that come from Paris and Brussels. The high speed trains are run using a totally different way of signalling trains than the old-style and conventional signals. The high-speed line is signalled using cab-signalling where the driver gets a display in the cab and that tells him when to stop his train, start his train and what speed he must run at. The trains travel up to 186 mph, and that's just too fast for the driver to be able to see signals out on the track. All the systems, whether you're in a lever box or you're in this type of modern technology it's all designed to fail safe and that is any failures, the signals go back to red. This job carries a lot of responsibility. You are responsible for people's lives on the trains, the public, drivers, track workers. You do have a fair bit of responsibility. No matter how much the technology changes, the one thing that remains the same is the safety and the security of the trains out on the track.


Basic concept

The basic problem for train control is the relatively long stopping distances of a loaded train. This is often far longer than the operator's eyesight, especially at night or in bad weather. The distances are great enough that local terrain may block sighting of trains ahead, and even the routing of the rails, around bends and such, may make it difficult to even know where to look for another train.

This leads to the possibility that a train may break down on the tracks, and the following train suddenly comes upon it when rounding a bend, or suddenly sees its rear signal lamp. In these situations there will not be enough room for the train to stop before it collides. This is known as the "brick wall criterion". Even in the case of two fully operational trains, differences in speed may be great enough that a faster train may not have time to slow down to match the speed of the one in front before it overtakes it.

Blocks avoid these problems by ensuring there is a certain minimum distance between trains, a distance that is set to ensure any train operating within the speed and load limits will have time to stop before reaching a train ahead of it. There are many potential solutions to implementing such a system.

Block signalling methods

Strict timetable operation

Most rail routes have a sort of natural block layout inherent in the layout of the railway stations. This provides the ability to implement a set of blocks using manual signalling based at these locations. In this case, the station operator places a flag indicating a train has just left the station, and removes it only after a fixed time.

Trains operate according to a strict timetable, and as such, cannot leave a station until an appointed time, and until any other trains they were to meet at that station have arrived. If one train is delayed, all trains it is scheduled to meet are delayed. This can quickly lead to all trains on the railway being affected.

This method is not authorised for use in many high-traffic railway systems

Timetable and train order

Popular on single track lines in North America up until the 1980s, Train Order operation was less a block system and more of a system of determining which trains would have the right of way when train movements would come into conflict. Trains would make use of a predetermined operating plan known as the timetable which made use of fixed passing locations often referred to as stations. Amendments to the operating plan would come from a train dispatcher in the form of train orders, transmitted to the trains via intermediaries known as agents or operators at train order stations.

This method is not currently authorised for use in the UK. A similar system, known as Telegraph and Crossing Order, was used in the 19th century, but after three serious head-on collisions in the 1870s (Menheniot, Cornwall Railway, 1873; Thorpe, Great Eastern Railway, 1874; Radstock, Somerset & Dorset Railway, 1876) its use was condemned.

In North American train order system was often implemented on top of other block systems when those block systems needed to be superseded. For example, where manual or automatic block was implemented, train orders would be used to authorize movements into occupied blocks, against the current of traffic or where no current of traffic was established.

One train working

One train working (with train staff)

If a single track branch line is a dead end with a simple shuttle train service, then a single token is sufficient. The driver of any train entering the branch line (or occupying any part of it) must be in possession of the token, and no collision with another train is possible. For convenience in passing it from hand to hand, the token was often in the form of a staff, typically 800 mm long and 40 mm diameter, and is referred to as a train staff. Such a staff may be a wooden staff with a brass plate stating the section of line on which it is valid, or it may be in the form of a key.

In UK terminology, this method of working was originally referred to as One Engine in Steam (OES).

One train working (without train staff)

A modern variation of the One Train Working system operates without any train staff. On these lines the clearance of the controlled branch entry signal is the driver's sole authority to enter the branch, and once the train has passed that signal, the interlocking will hold it at 'danger' (and the signal cannot be cleared a subsequent time) until the branch service train, on its return journey has sequentially operated two track circuits at the start of the branch. Continuous train detection on the branch is not required. Safety is ensured by the interlocking circuitry, and if a track-circuit failure occurs then special emergency working by pilotman must be introduced.

Accepting a token on the South Devon Railway
Accepting a token on the South Devon Railway

Token block

Authority to occupy a block is provided by physical possession of a token by train staff that is obtained in such a way that ensures that only one train is in a section of track at a time.

Ordinary train staff and ticket (OTST) or (OTS&T)

Ordinary train staff sections

Some low traffic lines dispensed with Tickets and became Ordinary Train Staff sections (OTS).

Electric train staff (ETS)

These came in two sizes, large and miniature.

Manual block system

Authority to occupy a block is conveyed to trains by the use of wayside signals manually controlled by human operators following various procedures to communicate with other block stations to ensure separation of trains.

Telegraph block

Used on multiple track sections whereby the passage of trains from one point to the next was controlled by instruments connected by telegraph wires. Used extensively in Australia.[2]

Telephonic block

In this system, the occupation of a given section of track between two stations is agreed between its station masters, via telephone. For greater safety there can be additional layers of protection; for example, a regulating post, with supervisory powers connected to all the stations in a line; timetable (Portugal); and/or computer assistance (France).

Portugal, Spain and France still use this system on at least some main lines, although the total length of track governed by this system is decreasing rapidly due to its labour intensity and its inherent perceived lack of safety, relying as it does primarily on human communication (sometimes involving more than just the two station masters at each end of the block) and simple railway interlockings at the stations.

In Portugal, the telephonic block was the main safety system across the national railway network until the mid-1990s due to lack of resources. Thus, it evolved to try to provide multiple layers of safety on busy single-track lines with diverse train types, albeit at the cost of high levels of staffing. In the Portuguese system, although the authority of train movement on the main lines is the sole responsibility of the stations along those lines, a regulating post oversees them and, in case of disagreement, instructs stations as to how the traffic should be organised. On the other hand, each train timetable indicates all interactions with other trains (e.g. crossings with other trains; trains that they overtake; trains that overtake them) clearly marked at the stations at which those interactions should occur. Any deviation from that—arising, for example, from delays or extra trains—must be provided to the train crews in writing. Despite the general practice that, when two trains cross, they both stop at the nearest station, this system allows for good average speeds for fast trains similar to those on an automatic-signalling line. However, if minor delays occur and then proliferate, longer delays can arise as the system's additional safety mode is invoked (i.e. the paperwork-intensive process of updating train-movement instructions to reflect the altered crossing patterns). Such delays would not happen, at least not for the same reason, on an automatic-signalling line.

In general, the system dictates that a block is assumed to be closed; that is, permission must be obtained before a train is allowed to enter a block at one station en route to the other. However, in France, on multiple tracks, the block is usually open in unidirectional track sections. That is, after a station confirms that a previous train has vacated the block, the next train travelling in the same direction can immediately enter the block, with the station master at the entry station informing the exit station of the time that the train entered the block.

Tokenless block

This a system for use on single track railways, which requires neither the use of tokens nor provision of continuous train detection through the section. The signalling is designed such that the controlling signals will only allow one train to enter the line. The signalman at the far end of the section must visually check that the whole train has left the section and not become divided.[3]

Automatic block signaling

Vertical colour light signal on the Enshū Railway Line in Japan
Vertical colour light signal on the Enshū Railway Line in Japan

Automatic block signaling uses a series of automated signals, normally lights or flags, that change their display, or aspect, based on the movement of trains past a sensor. This is by far the most common type of block system As of 2018, used in almost every type of railway from rapid transit systems to railway mainlines. There are a wide variety of systems, and an even wider variety of signals, but they all work in roughly the same fashion.

Like the manual block systems outlined above, automatic systems divide the route into fixed blocks. At the end of each block, a set of signals is installed, along with a track-side sensor. When a train passes the sensor, the signals are triggered to display the "block occupied" aspect on the signals at either end of that block. In most systems the signals do not immediately return to the "block empty" aspect when the train leaves, instead there is some sort of mechanical delay that retains the block occupied aspect, or more commonly, presents a "proceed with caution" aspect.

Moving blocks

In terms of ensuring safety, the real consideration is the stopping distance of a given train and the distance at which it can spot another train. Blocks do not actually implement this concept, they implement a signalling system that ensures the worst performing train on a line has enough time to stop. This means any train with better stopping performance is forced to operate at speeds that are lower than its maximum, unless all of the trains on a particular line are identical.

The key issue is that a given train cannot safely see another train in time to stop. However, this is not true for trains that are equipped with some sort of inter-train communications system. In this case, any given train can keep itself at a safe distance from other trains, without the need for fixed blocks. These moving block systems have become popular since the required technology first started appearing in the 1970s.

In such systems, any train on the route can listen for signals from all the other trains, and then move in a way to ensure they have enough distance to stop. Early moving block systems used a cable strung along the rail line. Trains would use magnetic inductance to inject signals into the line indicating their location. The cable could also provide that location in a variety of ways that could be picked up by a sensor on the train. More modern systems may use off-board location systems like Global Positioning System or track-side indicators, and send the data between the trains using various radio-based methods.

The advantage to moving block systems is that there is no fixed number of trains on the line, because there are no fixed blocks. This can greatly improve route capacity, as seen in the Jubilee line and Northern line on the London Underground, where upgrades for the 2012 Summer Olympics improved capacity by about 50%.[4]


Block working was proposed by William Fothergill Cooke in 1842 in Telegraphic Railways or the Single Line as a safer way of working on single lines. Previously, separation of trains had relied on strict timetabling only, which was unable to allow for unforeseen events. The first use of block working was probably in 1839 when a Cooke and Wheatstone telegraph was installed in the Clay Cross Tunnel of the North Midland Railway. Instruments specific to block working were installed in 1841.[5]

See also


  1. ^ This is assuming the trains themselves are not changing. A counterexample is the Advanced Passenger Train, which was specifically designed with powerful brakes in order to operate at high speed within what was a relatively low-speed block layout.


  1. ^ "BLOCK SYSTEM".
  2. ^ "Bulletin". Australian Railway Historical Society. March 1961. pp. 43–51.
  3. ^ "Railway Group Standard GK/RT0051" (PDF). p. D1. Archived from the original (PDF) on 2008-11-20.
  4. ^ Mylius, Andrew (9 October 2003). "Moving block signals finally go ahead on Jubilee Line". New Civil Engineer.
  5. ^ Kieve, Jeffrey L., The Electric Telegraph: A Social and Economic History, pp. 33-34, David and Charles, 1973 OCLC 655205099.
This page was last edited on 10 July 2019, at 18:35
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