Release of train route A train route can be released in one of two ways, either automatically or manually.
Automatic release is performed by the interlocking system and happens once a train has completed its train movement.
Manual release is performed from the operator’s panel. It can be performed even if a train is already on the route, but then the train no longer has the same measure of protection. Manual release of train routes is not considered.
2.5 Interlocking System
An interlocking system is a mechanical, electrical, electronic or hybrid system that controls physical objects, such as signals and points, in a limited area [10].
The Danish railway network is controlled by several different generations of interlocking systems, ranging from the very old mechanical, over the electrical relay based systems of the ’50s and ’60s to the newer computer based electronic systems.
This work considers the 54 type relay based interlocking system, since this is the type that is used at Stenstrup.
The interlocking system has three major fundamental tasks, which is tocontrol, supervise andensure safety.
Control Manuel or automatic operation of points, signals etc.
Supervision Continual supervision of the state of points, signals, isolation etc.
Ensuring Safety Preventing points, signals etc. to be operated in such a way that it allows conflicting train movements.
2.5.1 Relay Based Interlocking
In a relay based interlocking system, the rules of the interlocking system is implemented in electrical circuits. A circuit consists of components, such as
20 Domain Description
relays, contacts and buttons, that are connected by wires and powered by a power supply.
2.5.1.1 Relays
A relay is an electrical component that can be in two states, drawn (symbolised with↑) and dropped (↓).
When power is supplied, the relay will be energised. This activates the electro-magnet, which draws the armature so that the upper contacts become connected while the lower become disconnected (fig. 2.10a).
When the power is cut, the magnetic field will disappear and gravity will cause the armature to drop. This again switches the active connections (fig. 2.10b).
Electromagnet
+
-Armature
Contacts
(a) Relay drawn
+
-(b) Relay dropped
Figure 2.10: When a current energises a relay, its electromagnet is activated.
This pulls the armature, which creates a connection with the upper contacts (a).
Without power, the armature is dropped, which creates a connection with the lower contacts instead (b).
Other types of relays exists, for example steel core relays, which are able to retain their state even after losing power.
2.5.1.2 Logic of Relay Based Interlocking Systems
The state of the physical objects can be captured by relays. By assigning a relay (and sometimes multiple replays) to each physical object on the station, the
2.5 Interlocking System 21
state of the entire station is captured. This can then be use to control the train movements in a safe manner.
Each track section has a relay associated (called a track relay), which is drawn when the track is considered vacant.
Each points section has two additional relays associated. One is drawn when the points section is locked in the plus position and the other is drawn when the points section is locked in the minus position.
In addition there are relays capturing the state of route locking, route releasing, each lamp on each signal etc.
Buttons can be pushed and released. When a button is pushed, the current is allowed to flow through, while the connection is cut when the button is released.
A relay based interlocking system consists of two parts, theinternal system and the external system. The internal system is therelay circuitry that is controlled by the interlocking system, e.g. the signal relays. The external system on the other hand, consists of the circuitry that is controlled by the environment, e.g.
points relays, and train detection relays.
The logic of the system is created by arranging the connections of the components in a certain way. Serial connections create conjunctions, while parallel connections create disjunctions, as the current will follow whichever path it is allowed (fig.
2.11).
The situation on fig. 2.11acan be translated to the following boolean expression L=A∧B, where L is true when the lamp is turned on and A and B are true when the respective button is pressed. Similarly the situation on fig. 2.11bcan be translated toL=A∨B, meaning either button A or B must be pressed for the lamp to turn on.
Circuit Diagrams The circuit system is documented in circuit diagrams as part of the station documentation. The diagrams are divided by functionality, such that one diagram may show the route locking, while another shows the route release. The diagrams always shows the system in the normal state. The normal state is the state the system is in when every track section is vacant, no train routes are locked, the points are in their initial position and so on.
The system is operated from the operator’s panel.
22 Domain Description
A
B L
(a) Conjunction
A B
L
(b) Disjunction
Figure 2.11: Serial connecting components create a conjunctive relationship between them, as seen in (a) where buttons A and B are serial connected to the lamp L. When both buttons are pressed, the current will be able to pass through and turn on the lamp. (b) shows a situation where buttons A and B are connected in parallel to the lamp L. The lamp will turn on if either A or B is pressed.
2.5.1.3 Operator’s Panel
The operator’s panel shows the layout of the station and is equipped with buttons and lamps. Some buttons are used to switch the position of the points, some are used to lock routes (fig. 2.12), while others again have different purposes. The lamps indicate the state of track sections and points.
Figure 2.12: Diagram of operator’s panel for Stenstrup station.
Switching Points The position of a points section is switched by pressing and holding the + or−button next to the points section. When the position has been
2.5 Interlocking System 23
locked in either the plus or minus position a lamp will lit on the corresponding branch.
Locking Routes A route is locked by holding the button for the start point and the end point. The buttons are marked I, U and T. Button I is used to indicate the start of an entry route, while button U is used to indicate the start of an exit route. U or I must be pressed in combination with a T button, which indicates the end of the route.
2.5.2 Train Detection Equipment
Different methods of track detection are adopted throughout the world. Bane-danmark uses track circuits.
2.5.2.1 Track Circuit
A track circuit is a device that detects the absence of trains.
Each rail is connected to a power supply in one end and a relay in the other end.
Since the rails are conductive, the relay will be energised by the current running through the rails (fig. 2.13a). Since train wheels and axles are conductive as well, a train on the track section will cause a short circuit, thus cutting the power to the relay. Without power, gravity will cause the relay to drop (fig. 2.13b).
+
-Current Flow
(a) Relay drawn
+
-(b) Relay dropped
Figure 2.13: As long as the track section is clear, the current can travel through the rails and energise the relay (a). A train on tracks, on the other hand, will cause a short circuit, preventing power to reach the relay, which will drop (b).
24 Domain Description
As a consequence of this design, the relay will drop in case of power outage as well. This fail-safe feature contributes to the popularity of this method of train detection.
The track sections are isolated so that each section forms it own circuit. In this way the location of trains can be detected in discrete units.