Colour Light Signalling Flashcards
What is the purpose of fixed signals in railway systems?
Fixed signals divide the track into sections to ensure that only one train is permitted to enter one section at one time.
What is 2 aspect signalling?
2 aspect signalling allows only 2 aspects to be displayed in each signal, similar to the semaphore arms they replace.
What aspects can be displayed in 2 aspect signalling?
Distant signals can display yellow or green, while stop signals can display red or green.
What is a block section in 2 aspect signalling?
The block section is the distance between two stop signals.
What is 3 aspect signalling?
3 aspect signalling allows for greater control over train movements by displaying green, yellow, or red aspects.
What is a block section in 3 aspect signalling?
The block section is the distance between two signals on a 3 aspect system.
What are the normal signalling rules for 3 aspect signalling?
A stop signal must have a yellow signal before it. If a signal shows a green aspect, the next signal must also show a proceed aspect (green or yellow).
What are 4 aspect signals used for?
4 aspect signals are used on higher speed lines or where the sections between signals are short to give advanced warning of stop signals ahead.
What additional signal does a 4 aspect signalling system display?
A 4 aspect signalling system can display a double yellow used as a preliminary caution that the next signal is yellow.
What is the future of signals?
The future of signals is LED.
How can a 4 aspect signal be condensed?
A 4 aspect signal can be condensed into 2 aspects, with the bottom aspect being green, red, or yellow and the top one displaying another yellow for preliminary caution.
What are the three main categories of controlling colour light signals?
The three main categories are: 1. Controlled - Operated manually by the signaller or set by the signaller to work automatically. 2. Automatic - Controlled by the passage of trains through a section. 3. Semi-automatic - Controlled automatically by the passage of trains but can be controlled manually from a signal box or ground frame.
What happens when the track circuit is unoccupied?
The current from the power source travels around the circuit, passing through the signal relay, which keeps the green proceed aspect signal light open.
What occurs when a train occupies the section of the track circuit?
The electrical current is short circuited across to the other rail due to the electricity taking the path of least resistance.
Why does the electrical current stop passing through the signal relay when a train is present?
There is less resistance to pass through the train wheels than through the signal relay.
What happens when the electrical current stops passing through the signal relay?
The magnetic field holding the switch open ceases, causing the switch to close and the red stop signal aspect to be displayed.
What are axle counters used for?
Axle counters are used to count the number of train wheels that enter a section.
Where are axle counters fitted?
Axle counters are fitted near the railhead.
What mechanisms do axle counters use?
Axle counters use mechanical treadle or magnetic switch.
When will the signal show a proceed aspect?
The signal will show a proceed aspect only when the same number of train wheels are recorded by axle counters at both ends of the section.