Why do railway line traffic lights have 4 lights and what colour are they?

Question

Why do railway line traffic lights have 4 lights and what colour are they?

Answer 1

Railway signals come in two types. Semaphore, which is the old arm sticking out at an angle which is called Absoloute Block. Track Circuit Block, which is the coloured traffic light system is now more common in the UK.

The light come in multiple forms from 1 Aspect LED (the LED changes colour) to 1 Aspect Stop (Red), 2 Aspect (either Red / Green or Green / Yellow), 3 Aspect (Green, Yellow, Red) or 4 Aspect (Yellow, Green, Yellow, Red). Depending on weight of traffic over the sectional area depends on the number of aspects and space apart.

On top of all these signals there are a number of Ground Frame, Shunt and Route signals which complicate the system even further.

Answer 2

Introduced when electric driven trains were able to reach higher speeds on the UK pre world war2 Southern Railway.

A three light system can only warn of a red light two signal places ahead along the line.

Green means that the next signal can be green or double amber.

Double amber means that the next signal is amber.

Amber means that the next signal is red.

So it gives an extra warning stage.

Of course the line may partially clear as the train moves towards its next signal point, and the situation improves to giver a safer indication.

I'm sure that somewhere I've seen double greens, meaning that the next signal could be double green or single green, giving an even hint of a red light ahead.

Answer 3

Red= stop

Single Amber means= The next single is Red put the breaks into second phase IE stopping the train

Double Amber= Run the train at caution as signals ahead could be red so prepair to stop by putting on the breaks so reducing the trains spead

Green= Line is clear so train can travel at line speed.

Answer 4

I cant answer for the U.K. but in the States we have red, yellow, green and clear (white) or "lunar"

They all convey different meanings regarding safe movement of trains.

The combinations of lights and flashing lights is staggering and will vary from one railroad to another, indeed from one subdivision to another.

For instance a yellow over yellow signal on one of our subdivisions is called an "approach diverging", across an imaginary line at another subdivision the exact same signal is called an "approach medium" and the meanings are similar but different in some aspects.

So, the short answer is there is no short answer lol.

There is quite a long section in our rule books pertaining to signals and their use, gets complicated and varies from carrier to carrier and they type of territory the trains are operating in.

Much more involved than traffic lights.

Answer 5

A railway line is separated into sections. Only one train is allowed on to each section at a time, for safety.

A red signal means there is a train in the next section, therefore the driver must stop.

Yellow means the next section is clear.

Double yellow means the next two sections are clear.

Green means the next three sections are clear.

Answer 6

This is Multiple Aspect Signalling (MAS) which was introduced on British Rail in the 1960's to cope with high-speed running.

The colours are:

1. Red = danger, stop;

2. Yellow = caution, proceed slowly, next signal at danger;

3. Double yellow = caution, next signal is yellow;

4. Green = line clear, proceed.

The colours are in the following sequence (from bottom to top): red, 1st yellow, green, 2nd yellow.

NB. Important difference between the railway yellow and the road traffic light amber is that the latter means 'stop', but a train may pass a yellow/double yellow at reduced speed.

The system is thus arranged because it is felt that at speeds of 100mph+ it is safer for the driver to have 2 warnings instead of 1 to stop.

At junctions it is also normal to have 1 signal with a bar of white lights at an angle above it to indicate when a train is being directed onto a diverging route.

An alternative to this is at a station where a train may be directed onto one of several tracks and an illuminated number below the signal indicates which track.

Answer 7

Hooo boy.

In the US, each old railroad had somewhat different types of signals, and in the 1970's many railroad companies merged into several super large railroad companies, but that simply meant that now one railroad had a hodgepodge of many signal types.  This is still the case today.

In general, the traffic signals for an automobile tells the car whether to stop or go - in railroad lingo, it is called "giving the car authority to move."  On the other hand, railroad signals not only tell the train whether to stop or go (authority), but also what the condition of the track ahead is (is there somebody in front of us?  How close?).  In some systems, the signal will also tell the train which rail path it is to take (directional indication).  Just imagine if traffic lights for your car had to tell you not only whether to stop or go, but also if someone is ahead of you within a mile, and whether you were supposed to change lanes.  Pretty complicated, so it's why railroaders have to be pretty quick thinking, and have very good eyesight.

In general, if you see all lights green, the track ahead is free and you can go.  Red on top with some other color underneath  means you are going to go onto a different track, but keep going.  Yellow on top means to slow down, and be expecting to stop at a future signal.  All lights red means to stop, if it's at a junction, or just go really slow - there might be a train ahead.

Answer 8

Please also note that on the London Underground,  normally there are only "Two-aspect" signals where a "stop" signal can show either a red or a green (Stop or go) - or a "Repeater" signal a yellow or a green (yellow - the next stop signal is showing red, or green - so is the next stop signal) - and these repeater signals are only used where visibility is restricted - curvature in a tunnel section, bridges  trees or other obstructions in open sections, then there are "Fog Repeaters" which show green or 'lunar white' and yellow, obviously used only in open sections of line.

Sometimes - especially on the Met Line there are three aspect signals - red yellow and green (from the bottom up) to give additional warning of a red signal, especially to drivers of trains out of Marylebone on the Chiltern lines.

Note that on the L.U. it is possible in places to see two greens on a signal - this is not quite what it seems at first glance - it is simply a stop signal AND a repeating signal mounted on the same post, and the repeater will only show an aspect if the stop signal is showing green - this is to avoid confusing drivers with a red AND  a green together which COULD be possible if the stop signal is a 'controlled signal' (i.e. by a signal operator) and the next signal is an automatic one which could normally be showing a green.

Complicated affair, railway signalling, isn't it - and of course, the red light is always at the bottom of a display so that it cannot be obscured by snow building up on the sun shades ! ! !

Answer 9

There is also a fifth signalling aspect (on the Great Western lines) of a single flashing yellow which warns that the following signal is showing a "double yellow" aspect. This is provided to aid drivers of high speed trains which need additional braking distance to stop.

Many three-aspect (red, yellow and green) signalheads are now being changed to a single-lens LED display.

Answer 10

4 aspect signals (traffic lights) are on high speed running lines and are spaced approx 1 mile apart being able to keep maximum number of trains running but at same time allowing the driver to bring the train safely to stop from high speed. In running order it is green, double yellow, yellow and red. Over certain speed the green will flash giving the driver authority to run at the higher line speed.