Why do some railroad signals have 2 sets of signals in the same direction.?

Question

Why do some signals they have two sets of red yellow and green lights facing in the same direction what does it mean and what is the purpose.

Answer 1

One signal head can generally only display three aspects (displays): red, yellow, or green usually.

You need a lot more meanings than that to tell a train what it needs to know.  

- Stop.  Something is bolluxed up on the track ahead.

- Approach. The next signal is probably "stop" so set brakes!

- Clear.  Track looks clear ahead.

There's our three.  Now we've run out of aspects.

- Advance approach.  The next signal is probably "Approach", so start setting brakes if you're on a fast train.

Hmm.  What aspect can we use?  Some railroads flash the yellow light.  Others put up a second signal head, so now you have 2 heads for 3x3=9 aspects.  (yellow over green is a common aspect for "advance approach".)

Things really get hairy at crossovers.  Now there are several directions a train could go, and the signals must convey all of the above plus speed restrictions.   That's why you often see THREE signal heads (3x3x3=27 possible aspects).  

Here's a page on how several railroads did it.

http://www.spikesys.com/Trains/Signals/

As you can see, the Pennsy and B&O really went nuts with their signals. This became a problem when rampant mergers made it necessary for an engineer to be fluent in a half dozen signal systems!

Answer 2

In the UK it used to be the practice at the approach to a junction to have 2 sets of signals, 1 for each diverging line. These have now largely been replaced by 1 signal with a route indicator.

The route indicator consists either of a set of white lights above the signal to show the direction of travel or a number below the signal showing the track to which the signal aspect applies.

Answer 3

There are aspects that require multiple signal heads.  There are a variety of aspects, including, red over green, red over yellow, double red, red over lunar, etc.  There are many combinations.  These are usually "controlled" signals, wherein the dispatcher can control aspect display, turning them red or clearing a red aspect, that were once known as "absolute" signals.  At a controlled signal displaying a red aspect, with the indication of "stop," before proceeding, the signal must display an aspect other than red, or permission to pass obtained from the dispatcher, or "operator" when within interlocking limits.  When a train dispatcher grants "track and Time," for example, he can turn the opposing controlled signals red, to provide protection within the track and time limits.  

When an automatic block signal displays a red aspect, the indication is "stop and proceed," unless there are other actions mandated by the carriers special instructions, such as when the signal is a "P" or, "protective signal" used in connection with another device, such as a slide fence or high water or collision detectors.

They are usually used where there is a diverging route to be used, such as entering a siding in CTC, on a red over yellow signal.  In essence, it means "red" for the main, and "yellow" for the siding.  A diverging route can be another main track as well.

Signal masts or signal bridges have three signal heads when required as well, often times with a smaller head underneath all, to be able to display a "lunar" aspect.  Searchlight type signal heads can only display a maximum of three aspects per unit, so where various combinations are required, it takes more than one signal head to do the job.  A good example is to be found on the Black Butte Sub on my own seniority district.  At Balck Butte, when running north on the main track, which is CTC territory, there is a signal mast with four searchilght type heads on it.  All red indicates stop.  Red over yellow over red indicates movement into the north siding.  Red over red over yellow or green indicates movement onto the main track of the CORP (California Oregon Pacific) short line.  This used to be the Siskiyou Sub before being shortlined.  There is a fourth, smaller unit, below all three heads so that a "lunar" aspect can be displayed.  The reason is, as stated, there are only three aspects an individual unit can display.  With respect to the bottom searchlight signal, it has to be able to display red, yellow and green.  The extra unit is added to be able to display a lunar aspect (lunar is kind of an opaque bluish / whitish color).

Good question..........

Addendum:  Hey, Brother Bear.  Point taken.  Thank you.  But don't sell short the folks who pose questions here regularly, like Kenster1.  They often ask advanced questions and are quite capable of comprehending a detailed answer.  If not, any confusion can be clarified by another question or, in most cases, a following answer can clarify.  

It has been my experience, if one really wants to take the current pulse of the nation's railroads, ask a railfan.  I've often gotten better line-ups from them than from the crew dispatcher's office..................

Answer 4

I am comfused it sounds like you are talking about a traffic light.  I've vever seen a railroad signal with red, yellow, and green lights.  Unless it is a traffic signal near a track then sometimes the lights are syncd' to flash red

Answer 5

Do any of the people with these long *** answers work on a railroad? If you do you have way to much time on your hands! What are you going to talk about next? Time operated switches compared to electric locks????

Answer 6

Hey Hog, From one hog head to another, your getting way to technical for the average Joe to understand if you don't do this for a living. Make it short and sweet and in layman's terms and they might see what your trying to say.