15-11-2012, 11:11 AM
SIGNALLING AND OPERATIONS
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Railway signalling is a system used to control railway traffic safely, essentially to prevent trains from colliding.
Timetable operation is the simplest and earliest form of operation control of a train system . A fixed schedule is drawn up with which every train crew must be familiar. Trains may only run on each section of track at their scheduled time, during which they have 'possession' and no other train is permitted to use the same section.
With the advent of the telegraph in 1851, a more sophisticated system became possible because this provided a means whereby messages could be transmitted ahead of the trains. The telegraph allows the dissemination of any timetable changes, known as train orders.
SIGNALLING BLOCK SYSTEMS
To avoid collision of trains railway tracks are divided into sections known as blocks. In normal circumstances, only one train is permitted in each block at a time.
In manually control blocks before allowing a train to enter a block, a signalman must be certain that it is not already occupied. When a train leaves a block, he must inform the signalman controlling entry to the next block.
Fixed blocks include the section of track between two fixed points . A train is not permitted to enter a block until a signal indicates that the train may proceed. When calculating the size of the blocks Line speed, Gradient , Sighting and Reaction time of the driver.
Under Moving block systems, computers calculate a 'safe zone' around each moving train that no other train is allowed to enter. The system depends on knowledge of the precise location and speed and direction of each train, which is determined by a combination of several sensors: active and passive markers along the track and trainborne tachometers and speedometers (GPS systems cannot be used because they do not work in tunnels.)
AUTOMATIC TRAIN CONROL
It is a train protection system for railways, ensuring the safe and smooth operation of trains
It uses Communication-based train control (CBTC) which uses radio transmissions , GPS systems etc
the system includes ATP (Automatic Train Protection), ATO (Automatic Train Operation) and ATS (Automatic Train Supervision).
It is implemented in most metro systems in the world such as Tokoyo Metro in different versions .
Advantage
make possible the use of cab signalling instead of track-side signals
use of smooth deceleration patterns in lieu of the rigid stops encounters
The basic safety requirement, to keep trains a safe distance apart, is performed by the ATP, which has a control unit for each block. This control unit receives the data from the blocks ahead, converts that into a speed limit for the block it controls and sends the speed limit data to the track
This data is sent to the ATS computer where it is compared with the timetable to determine if the train is running according to schedule or is late or early. To adjust the train's timing, the ATS can send commands to the ATO spots located along the track.
The basic function of ATO is to tell the train approaching a station where to stop so that the complete train is in the platform. This is assuming that the ATP has confirmed that the line is clear.
AUTOMATIC TRAIN OPERATION
The ATO spots, which can be short transmission loops or small boxes called beacons or "balises", give the train its station stop commands. The spots usually contain fixed data but some, usually the last one in a station stop sequence, transmit data about the time the train should stop (the dwell time) at the station and may tell it how fast to go to the next station (ATP permitting).
The train approaches the station under clear signals so it can do a normal run in. When it reaches the first beacon - originally a looped cable, now usually a fixed transponder - a station brake command is received by the train
ATO DOCKING AND STARTING
ATO also controls door operation and restarting of train from a station.
When the train has stopped, it verifies that its brakes are applied and checks that it has stopped within the door enabling loops. These loops verify the position of the train relative to the platform and which side the doors should open. Once all this is complete, the ATO will open the doors
After a set time, predetermined or varied by the control centre as required, the ATO will close the doors and automatically restart the train if the door closed proving circuit is complete.
Once door operation is completed, ATO will then accelerate the train to its cruising speed, allow it to coast to the next station
TRAIN DETECTION
TRAIN CIRCUIT -- The rails at either end of each section are electrically isolated from the next section, and an electrical current is fed to both running rails at one end. A relay at the other end is connected to both rails. When the section is unoccupied, the relay coil completes an electrical circuit, and is energized. However, when a train enters the section, it short-circuits the current in the rails, and the relay is de-energized.
AXLE COUNTERS -- An alternative method of determining the occupied status of a block is using devices located at its beginning and end that count the number of axles entering and leaving. If the same number leave the block as enter it, the block is assumed to be clear
CENTRALIZED TRAFFIC CONTROL SYSTEMS
Centralized traffic control (CTC) is a signalling system used by railroads .
The system consists of a centralized train dispatcher’s office that controls railroad switches in the CTC territory and the signals that railroad engineers must obey in order to keep the traffic moving safely and smoothly across the railroad
In the dispatcher's office is a graphical depiction of the railroad on which the dispatcher can keep track of trains' locations across the territory that the dispatcher controls.
The majority of control points are located at electronically-operated switches which are remotely controlled or controlled manually by levers or switching mechanism
The most prominent feature of CTC is its signals. Signals govern movement over the section of track . A signal is placed where signal blocks meet. Separate signals are placed for trains travelling in opposite directions.
SIGNALS
Physical signals are erected at the lineside to indicate to drivers whether the line ahead is occupied and to ensure that sufficient space exists between trains to allow them to stop.
Mechanical signals – Older form of signals include a board that was either turned face-on and fully visible to the driver, or rotated so as to be practically invisible or a permanently-lit oil lamp with movable coloured spectacles in front that alter the colour of the light .
Colour light signals -- On most modern railways, colour light signals have largely replaced mechanical ones. Colour light signals have the advantage of displaying the same aspects by night as by day, and require less maintenance than mechanical signals.
Cab signalling –It is a system that communicates track status information to the train cab (driving position), where the engineer or driver can see the information