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Track Circuit Block (TCB) is a method of working that relies on continuous train detection throughout the length of every block section. The train detection equipment gives signalmen a continuous indication of the position of trains, avoiding the need for them to visually observe that every passing train is complete with tail lamp as an assurance that it has vacated the previous block section. Signalmen are free to admit a train into a block section at any time provided it is completely clear of vehicles. There is no need for signalmen at either end of a section to operate and observe block instruments as they do with the older Absolute Block system. Voice communication between adjacent signal boxes is provided, as is some means of passing train descriptions, such as by single-stroke bell or train describer. A train describer is a device that stores trains' class and routeing details and displays them to the signalman.
Track Circuit Block is the favoured method of working in modern signalling installations using lineside signals, the only drawback to its universal provision being the relatively high cost involved. Since there is no need to locate a signal box at the extremities of every section, any number of consecutive sections can be placed under the control of the same signal box. A large signalling centre may have more than a hundred miles of railway under its control, in which case a train describer is essential as it allows the signalmen to easily identify all the trains currently inside, and approaching, the areas under their control.
The Track Circuit Block system is equally suited to single line railways and has the advantage of not requiring drivers to obtain a token as authority to occupy a section. Entry into a single line section is governed solely by drivers' observance of the relevant stop signal; interlocking prevents trains being simultaneously admitted into the single line from both ends. If desired, additional stop signals can be installed within the single line to divide the section, so that a second train may enter the single line before the one in front has left it.
The name "Track Circuit Block" derives from the train detection equipment that is fundamental to the system having originally consisted of track circuits. These are electrical circuits in the running rails that detect the presence or, more accurately, the absence of vehicles within a portion of track. The limits of a track circuit are usually defined by insulated joints in the running rails, although 'jointless' track circuits are sometimes used. A power source connected at one end of the track circuit energises a relay connected to the other end. The relay is de-energised when the wheels and axles of a train place a short-circuit across the rails. Contacts of this relay control one or more repeating relays, the contacts of which can be used in interlocking circuitry or to operate track section indications in the signal box.
Axle counters are becoming increasingly common as an alternative form of train detection because of the benefits they offer when compared to track circuits, such as increased reliability. An axle counter system comprises detection points attached to the running rails at all the extremities of a track section, each of which is linked to an electronic 'evaluator'. A detection point has two transducers, each of which can detect a passing train wheel. Signals from both transducers are transmitted to the evaluator, which can determine the direction in which any passing wheel is moving. The evaluator stores in its memory a value equating to the total number of train axles currently inside the section that it supervises. As each wheel passes a detection point, the count for the relevant section increases by one if it is entering that section or decreases by one if it is leaving. Only when the count is at zero is the section presumed to be unoccupied. The evaluator drives a relay which, like a track circuit relay, is energised as long as the section is clear. The most modern evaluators can supervise multiple axle counter sections.
Consecutive block sections are divided from one another by stop signals, usually in the form of colour lights, although semaphore signals may sometimes be used. A stop signal is prevented from showing a 'proceed' indication unless the line ahead is proved clear by the train detection equipment for at least as far as the next stop signal (and, where required, the overlap beyond it) or buffer stop.
In Track Circuit Block territory, stop signals are categorised as 'controlled', 'automatic' or 'semi-automatic'. A controlled signal will not show a 'proceed' aspect unless the signalman has operated the corresponding lever, switch, or other device. An automatic signal is controlled by the passage of trains and will show a 'proceed' aspect as long as the section ahead is clear, no action by the signalman being necessary. A signal may be designated as automatic provided there is nothing but plain line (i.e. no points or crossings) throughout the section ahead (with the exception of trailing spring catch points) and there are no directly opposing routes. A 'semi-automatic' signal is one that usually functions as an automatic signal, but can if necessary be controlled. Semi-automatic signals usually protect features such as a ground frame controlled connection or a locally operated level crossing, within an otherwise plain line section. Before the ground frame or level crossing can be operated, the signalman must return the protecting signal to "danger". Semi-automatic signals ceased being provided in new works from 2002; current practice is to use a controlled signal in these circumstances.
|Fig. 1: Controlled, Automatic and Semi-Automatic Signals.|
A controlled signal may be provided with a facility enabling it to operate automatically for successive trains that are being routed along the same path. This avoids the signal having to be manually cleared for each train, so easing the signalman's workload. Most automatic signals have a facility that allows the signalman to replace their aspect to "danger" in an emergency; such provision is compulsory in new works. Some automatic or semi-automatic signals have an emergency replacement switch mounted on the signal post into which a special key may be inserted and turned to place its aspect to "danger".
Should a derailment occur in Track Circuit Block territory, leaving vehicles foul of an adjacent running line, traincrew can act to reduce the risk of subsequent collision by applying a track circuit operating clip across the rails. This causes the track circuit to be shorted out, placing or maintaining the protecting signals at "danger". Track circuit clips are ineffective with axle counters and to ensure an equivalent level of protection in the circumstances described, alternative safety measures are required where axle counters have supplanted track circuits as the system of train detection on a multiple track railway. Usually this takes the form of GSM-R radio, which a driver may use to initiate an emergency call to the signalman.
In the same way that Track Circuit Block can control the movement of traffic along a single line in both directions, a double (or multiple) track railway can be signalled to permit trains to run in either direction on any running line, resulting in greater flexibility. This is termed 'bi-directional signalling' (or 'reversible signalling') and it may be of the 'full capacity' or 'reduced capacity' variety.
Figure 2 represents a portion of double track railway with the additional signalling needed for full capacity bi-directional signalling coloured red. Signals 21 and 46 each have a position 4 junction indicator directing trains through the facing crossover and onto the opposite line in the wrong direction. The last wrong direction signal (numbers 24 or 45) has a position 1 junction indicator to route a train back onto the normal line via the trailing crossover. Within the area of bi-directional signalling, every signal has another one placed alongside it, applicable to the adjacent track. The effect of this is to allow wrong direction movements to run at a frequency similar to that of normal direction movements.
|Fig. 2: Full Capacity Bi-directional Signalling.|
Figure 3 shows a portion of double track railway with the additional signalling needed for reduced capacity bi-directional signalling coloured red. In this case, only the minimum of additional signals are installed, i.e. stop signals 64 and 77 and their associated distant signals, numbers 68 and 73. For signal 61 to display a 'proceed' aspect with a position 4 junction indicator, the line must be clear all the way to signal 77, the next stop signal in that direction. Thus, a portion of line that comprises several sections for trains running in the normal direction (four sections in this example, but typically there would be more) is treated as a single section for running in the wrong direction. This form of bi-directional signalling therefore allows wrong direction movements to run only at significantly lower frequency than in the normal direction. If the section is particularly long, additional wrong direction stop and distant signals may be installed at an intermediate location.
|Fig. 3: Reduced Capacity Bi-directional Signalling.|
'Simplified bi-directional signalling' (SIMBIDS) is a simplified form of reduced capacity bi-directional signalling that was implemented on some routes between 1988 and 2002. For reasons of economy, no AWS equipment is provided for signals that apply in the wrong direction and furthermore, the AWS equipment for normal direction signals is not suppressed for wrong direction moves, nor are any cancelling indicators installed. Lineside signs erected at the beginning and end of wrong direction running alert drivers to the commencement and termination of these special AWS conditions. The maximum speed for wrong direction running is limited to no more than 70 m.p.h. Wrong direction movements under SIMBIDS conditions are only permitted during possession of one line or during emergencies. It cannot be used for the regular passage of booked trains.