Stop Learning Signalling Without Quadrant Signalling System [Used From 1841]


This semaphore signal used in the form of Rectangular or fixed tails arms at the top of the fixed post.

The arm is kept in horizontal to attract the driver’s attention quickly as today Red led is placed in the height of the driver eye level.

The main aim is the ON aspect should visible first to the driver to ensure safety.

This mechanical arm arrangement of the semaphore signal is visible from very far in the daytime. It used to convey the information to the driver.

The arm can be placed as:

  1. The middle position below the horizontal
  2. The middle area of the above horizontal
  3. Straight horizontal line parallel with the ground
  4. Vertical position extends the post at the top
Quadrant Signalling System Diagram
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Quadrant Signalling System Diagram

This possibility could be on the Right-hand side or left-hand side of a Quadrant.  A fixed signal can operate at any one of these four quadrants of a circle. 

Railway Signalling depends on Quadrant logic, mostly in a signal schematic representation.

Since “Left hand” rule followed in India, left-hand side of Lower quadrant (LQ) and upper quadrant (HQ) used in Indian Railways. Based on this principle, Signals mostly placed on the left side of the track of an approaching train.

The arm in a lower quadrant has only two positions. One is horizontal, and another is the middle position on the left side. But in the right hand one other vertical position in parallel with extended post available.

Hence we have two types of quadrant signalling system are present.

  1. Lower Quadrant Signalling System
  2. Upper Quadrant Signalling System

Recommended: The Best technique to Start Learning Railway Signalling

Lower Quadrant Signalling System

Stop Signal

A lower quadrant signal can show only in two different positions.

One of them is horizontally parallel to the ground or can say 0°, and another one lowered to midway position or 45° in angle.

This different position is called “Aspect” in signalling concept.

So, here a maximum of two aspect signals can describe through this Lower quadrant Signalling System. The arm in the horizontal position (0°) will convey an aspect “Stop” indicating “Stop dead.”

And the arm lowered to the middle position (45°) will convey an aspect “Proceed,” indicating “Proceed,” i.e., cross the signal with permitted speed.

IndicationStop deadProceed
Aspect and Indication of Two Aspect Lower Quadrant Signal

As at night time, the arm is not visible, so to convey the information fixed light signal used. The red light used to denote “Stop,” where the arm is in horizontal and green light used to indicate the “Proceed” aspect when the arm is in 45° inclined midway.

The horizontal position or 0° angle is showing red light at night is considered as an “ON” aspect, and inclined arm or 45° showing green light at night considered as the “OFF” aspect in Signalling System. We almost use this ON and OFF aspect instead of using the indication of light.

Warner Signal

Two aspects signal minimum safety required as it contains less weight and attains low speed.

But when speed needs to high, the driver may not stop at the correct position due to driver can’t see the ON aspect from very far.

So, A warning needs to provide before a stop signal to covey the information that he is approaching a stop signal.

An advance information about ahead stop signal is provided by a warning to the driver.

This is achieved by placing a fixed signal called “Warner Signal”.

This signal precisely informs the driver that he is approaching a stop signal and also that he is required to stop or proceed.

Since the driver does not need to stop at a warning signal, as it is only pre-warned the aspect of the stop signal, this signal has to be different than a stop signal. It is a part of the lower quadrant signalling system.

AspectProceed with CautionProceed
IndicationProceed with caution and be prepared to stop at next  stop signalProceed
Aspect and Indication of Lower Quadrant Warner Signal

Since Warner signal is not a stop signal and red lighting light when “ON,” and this should be distinguishable than a stop signal in the night.

It is done by mounting the arm at a lower level in the post and provide a separate green light at 1.5 to 2 m above the arm.

When the signal lowered to the middle position, the red light changes to green light and the driver sees two green lights in post. Two important information is given to the driver through this warner signal

  • The arm is in horizontal during the day, and showing a red light within above green light in night indicates that the driver can proceed with caution but be prepared to stop at the next stop signal.
  • The lowering of the arm in the midway during daytime and showing two green light on the post in night time indicates that the driver can proceed with permitted speed and can expect that all ahead stop signal is in OFF aspect, i.e., he can run through mainline.

In some specific circumstances, to reduce the number of fixed signal posts, Warner Signal can be placed below the stop signal and the fixed green light dispensed with the Warner Signal aspect.

AspectStopProceed with cautionProceed  
IndicationStop deadProceed with caution and be prepared to stop at next  stop signalProceed
Aspect and Indication of a Warner Signal Place below a Stop Signal

The combination of two signals (Warner and Stop) in a single post gives the driver three indications in the 2-aspects lower quadrant signalling system.

  • Warner and stop both are in horizontal position and showing two red light for both signals, indicates “Stop dead” at this signal.                                                      
  • A green light showing above a red light indicates that he can proceed with the signal with caution and be prepared to stop at the ahead stop signal.
  • In third, both arms lowered to midway, and two green light is lightening. It indicates the driver that he can proceed with permitted speed and can expect all the stop signals are in the “OFF,” aspect, and also, the block section ahead is clear.

Advantage of a Warner Signal and Stop Signal in a post over A Warner signal in a Separate post

In the night time, if a separate warner signal showed green light above, a red light is in the ON aspect. But due to any reason, if the red light burned or extinguished, it is only showing a green light.

So the driver cannot identify this failure from far; instead of going cautiously, he goes with full speed where he must have to stop at the next stop signal.

As a result, he may not stop in a stop signal and may occur an accident if any train already at the station section.

So, to avoid this kind of possible movement, placing of Warner and Stop signal in the same post is the better way to ensure safety.

Although we also need to think about safety as well as Signalling System. Without ensuring proper protection, we cannot recognize any Signalling System.

Why Upper Quadrant Signalling System Needed?

Here, we all are trying to convey information to the driver by using Signalling System, but safety should adequately be ensure.

But from the driver point of view, sometime warner signal ON aspect does not signify the clarity of ahead stop signal.

If such information can provide, the driver can approach the signal confidently.

So to add some extra feature in the Signalling System, “Multiple Aspect Upper Quadrant Signalling System” introduced. It is also known as the Upper Quadrant Signalling System.”

Upper Quadrant Signalling System

Stop Signal

In the lower quadrant signalling system, the semaphore arm can be horizontal and lowered to midway position to indicate 2-aspects, but here one more area in parallel with the extended line of the post of the left-hand upper quadrant.

In this way, one more aspect is possible to obtain in the upper quadrant region. Hence, it is called the “Upper Quadrant Signalling System” modified version of the “Lower Quadrant Signalling System.”

In the Upper quadrant signalling system, more than two aspects called “Multiple Aspect” will present.

As this signal showing multiple aspects and present in the upper quadrant, it is also known as the “Multiple Aspect Upper Quadrant Signalling System (M.A.U.Q.)”.

The signal is required to convey three aspects, that three different lights Red, Yellow & Green, are fixed into the arm to carry the night aspect information.

IndicationStop deadProceed with caution and be prepared to stop at next  stop signalProceed
Aspect and Indication of M.A.U.Q. Stop Signal

The arm is in a horizontal position will convey the ON aspect indicating Stop dead. The night aspect of the horizontal position of the arm by showing a red light.

The rising of the arm “45° above horizontal” in the left hand of the Upper quadrant will convey the “Caution” aspect indicating the driver can proceed but cautiously and need to prepare to stop at the next stop signal.

Yellow light was lighted to showing the “Caution” aspect in the night.

The rising of the arm to 90° above horizontal with horizontal in parallel with the extended line of the post in a vertical position will convey an aspect “Proceed,” indicating the driver can proceed with maximum permitted speed, and the next stop signal is also “OFF.”

The corresponding night aspect is showing by a green light.

Distant Signal

As discussed in two aspects lower quadrant signalling system, the driver should warn before approaching the first stop signal of the station.

A signal similar to the Warner signal is needed to pre-warn the driver about his condition in the Upper quadrant signalling system.

This pre-warning signal is called “Distant Signal.” The term Distant used as an extreme signal from the station side on the approach side.

Upper Quadrant Distant Signal all Aspects
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Upper Quadrant Distant Signal all Aspects
IndicationProceed with caution and be prepared to stop at next stop signalProceed with attention and maybe stop in next to next signalProceed
Aspect and Indication of M.A.U.Q. Distant Signal

The mechanical arm has three positions- horizontal at 0°, 45° above horizontal, and 90° above horizontal. The arm is fish-tailed as warner signal.

ON” aspect is showing by the arm is in horizontal in the day, and the distant signal exhibits a yellow light in the “ON” position during night time.

The second aspect is indicating by the arm raised to 45° during the daytime. But in the night aspect, since the yellow light already used to show the “ON” aspect, a unique perspective is introduced by having two yellow lights one below to another.

This below light is placed in the post in such a way that in only this position (arm is in 45°) the driver only can see.

These indications describe the driver that the next stop signal is in “OFF”, but the driver can pass the stop signal with reduced speed. This aspect is called “Attention”.

A yellow colour light in the below of a post in the night time and fishtail shaped of the arm in the day time facilitate the driver to distinguishing a “Distant Signal” than a “Stop Signal” from long distance.


Before clearing a stop signal, the aspect of the next stop signal is also essential to ensure proper safety.

If ahead stop signal is in ON or Stop aspect, i.e., Red, then the previous sign must be in the “Attention” aspect, i.e., Yellow. “Proceed” aspect, i.e., green light, is only lighting when ahead also signal in the “Proceed” aspect—this perspective representation done by quadrant signalling system.

This safety is ensure by the Interlocking Principle.

Hence, the interlocking principle states that any signal aspect status depends on the next signal aspect.

It’s basically for safety, and we know trains guided in a specific path, it doesn’t have steering as other road traffic. So, the immediate stop of the train can derail the rail boogies, or the passengers can feel shaking inside the train.

To reduced shaking inside the trains, the declaration rate of the train should smoothly.

The driver starts to use brake from “Attention” aspect and finally stop at “Stop” aspect. So, hence an “Attention” aspect is very much needed before a “Stop” aspect and this rule is followed throughout the World.

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