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do model RRs need signals other than STOP, APPROACH, CLEAR?

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do model RRs need signals other than STOP, APPROACH, CLEAR?
Posted by gregc on Friday, December 8, 2023 5:35 AM

i've found basics signals to be surprisingly helpful on a large layout where the next block is hidden around a corner or inside a tunnel.

do model RRs need more than the 3 basic signals except to possibly more realistically model a particular prototype setting?

is there some value is route signalling to possibly confirm for a knowledgeable operator that switches aren't aligned properly?

greg - Philadelphia & Reading / Reading

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Posted by gmpullman on Friday, December 8, 2023 5:41 AM

My turnout leading off the main and into the staging yard will show a 'diverging approach medium' (Red over flashing Yellow over Red) when the facing points are set for the diverging route. I used a Circuitron FRED flasher unit to make the middle signal flash. As the engines pass the signal it will drop to red as the block becomes occupied.

It is handy for operators to know the switch is set for the yard.

Regards, Ed

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Posted by ATLANTIC CENTRAL on Friday, December 8, 2023 6:02 AM

Prototype signaling as you know is very complex and varies considerably depending on era and railroad.

There were many great modelers in the past who believed all that is not nessasary, or even valueable on our model layouts.

I use a streamlined signaling system on my ATLANTIC CENTRAL.

My signals are based around only using the absolute or interlocking (control point) signals of a CTC system.

You mention the term route signals, but with route signaling in CTC, the same signal also conveys all three pieces of information - route, occupancy and permission. This is actually easy to do on our models and is easy for operators to understand. and like Ed's example, my siginals confirm turnout position, and in my case confirm control authority.

Being a DC operator, signals need to instruct operators when they must stop, and when they can proceed.

I do have some intermediate signals that make the system appear to be more advanced than it is, but given the operation needs of a model railroad and our relatively short distances (even on a layout of considerable sze) I agree with those old timers like Ed Ravenscroft, there is no need for prototype level complexity - espeically for those modeling older historic systems.

My ATLANTIC CENTRAL is set in 1954, from what I understand there were few if any flashing indications in that era as just one example.

I would be happy to explain later what indications I do use and how they are generated if you are interested.

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Posted by gregc on Friday, December 8, 2023 9:00 AM

gmpullman
My turnout leading off the main and into the staging yard will show a 'diverging approach medium' (Red over flashing Yellow over Red) when the facing points are set for the diverging route.

this sounds useful.  without adding a 2nd head, flashing yellow can indicate that the switch is aligned for the diveging route and it is clear while yellow simply mean approach along the non-diverging route.

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Posted by ATLANTIC CENTRAL on Friday, December 8, 2023 4:45 PM

gregc

 

 
gmpullman
My turnout leading off the main and into the staging yard will show a 'diverging approach medium' (Red over flashing Yellow over Red) when the facing points are set for the diverging route.

 

this sounds useful.  without adding a 2nd head, flashing yellow can indicate that the switch is aligned for the diveging route and it is clear while yellow simply mean approach along the non-diverging route.

 

I could be wrong, but Ed's description sounds like a multi head signal?

Sheldon

    

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Posted by ROBERT PETRICK on Friday, December 8, 2023 5:04 PM

The signals on my layout indicate what's going on in the next two blocks, for either the main route or for the diverging route. Green indicates that the next two blocks (straight or diverging) are clear. Yellow indicates that the next block (straight or diverging) is clear, but the following block is occupied or the next turnout is thrown against traffic. Red indicates red.

Is this needed? Not really, I suppose.

LINK to SNSR Blog


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Posted by gregc on Saturday, December 9, 2023 3:42 AM

ATLANTIC CENTRAL
but Ed's description sounds like a multi head signal?

yes, understood

but i'm being pragmatic about what would be useful on a model RR without the expense of multi-headed signals.

again, yellow means APPROACH on the non-diverging track.   flashing yellow means the switch is aligned for the diverging route and is clear, that the block on the diverging route is un-occupied.

is this SLOW APPROACH, rule 288

412

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Posted by ATLANTIC CENTRAL on Saturday, December 9, 2023 9:05 AM

Not by my understanding, rule 288 implies two heads except as a dwarf in a yard or other slow speed situation.

Here is the thing, just because you put two heads on a mast, does not mean you have to actually light all the colors.

I have signals that have three aspects one of them never lights up.

Also, railroads often only installed two color heads where the third color would never be used.

I use type D signal heads (two or three vertical lights), a diverging route that is always speed restricted would only have yellow and red. The main route head above it would have green, yellow and red. 

If the turnout is set for the diverging route and the detection is clear on the diverging route the signal will display red over yellow. This confirms both pieces of information, the turnout is set for the diverging route, and that route is clear of traffic.

A train ahead on the diverging route would set the signal red over red.

The top head would stay red any time the turnout is aligned to the diverging route. The bottom head would stay red any time the turnout is aligned to the main route.

Only the head for the selected route would respond to occupancy conditions ahead.

To explain my comment above about asects that never light, at a typical high speed interlocking a crossover would be protected by two heads in the direction of travel thru the crossover.

I do not use approach aspects at control point signals based on a train in the second block ahead - that is too far away to be of use to the operators, they are going to see two more signals before they get to the second block ahead. 

So those are three aspect signals because they would be on the prototype, but they only show red over red, green over red, or red over green.

Here is the trap that you and others fall into - trying to match up the aspects you use to those "rule numbers".

Your operators are not going to learn that stuff. 

All they need to know is:

all aspects red = stop

top aspect green = main route clear

top aspect yellow - approach, train in 2nd block ahead

second aspect green or yellow = diverging route clear, yellow indicates train in second block ahead or restricted speed

Third aspect yellow, restricted speed diverging route.

All aspects not described in each situation are red.

It does not need to be more complex than that for our models.

To make my system look more realistic and provide more info to operators I have a signal half way thru each block - it looks like an automatic block signal, but it is actually an approach sginal for the next control point signal.

If the next control point signal is green, the intermediate (approach signal) is green. If the control point signal is red, the intermediate signal is yellow. The intermediate signals have three aspects but they never show red, just like the main route control point signals never show yellow, partly because of the DC control system, these aspects would convey no useful information to the operators.

Sheldon  

 

    

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Posted by gregc on Saturday, December 9, 2023 11:46 AM

ATLANTIC CENTRAL
Not by my understanding, rule 288 implies two heads except as a dwarf

right.   i now see that it's a single dwarf.   but ADVANCE APPROACH, rule 282-a is a sigle lamp flashing yellow.  Finally pulled out my book (Solomon).

but i hope you understand i'm not trying to be prototypical, just want to convey useful information to a modeler operating on a model layout.     if they intend to take a diverging route, they should see flashing yellow, not green or yellow

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Posted by ATLANTIC CENTRAL on Saturday, December 9, 2023 12:00 PM

gregc

 

 
ATLANTIC CENTRAL
Not by my understanding, rule 288 implies two heads except as a dwarf

 

right.   i now see that it's a single dwarf.   but ADVANCE APPROACH, rule 282-a is a sigle lamp flashing yellow.  Finally pulled out my book (Solomon).

but i hope you understand i'm not trying to be prototypical, just want to convey useful information to a modeler operating on a model layout.     if they intend to take a diverging route, they should see flashing yellow, not green or yellow

596

 

I understand, but in my view flashing is just extra complication. Are you thinking that because you only want to use one signal head?

I know your not trying to be prototypical, but if you look at the list of aspects I use, I simply use the most basic prototype aspects and ignore the complex prototype aspects. 

And I ignore permissive signals, except for my intermediate signals which give the appearance of permissive block signals between control points but require no extra logic because in function they are approach signals for the control point siginals.

We had this conversation once before, I don't understand why anyone would bother with signals and then try to define their function as a stand alone function and not link them?

But the linking can be basic, not complex like the prototype.

Even simplified, I think of it as a whole system, not just what a single signal does.

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Posted by gregc on Saturday, December 9, 2023 1:52 PM

ATLANTIC CENTRAL
Are you thinking that because you only want to use one signal head?

that's all we have

ATLANTIC CENTRAL
I don't understand why anyone would bother with signals and then try to define their function as a stand alone function and not link them?

don't understand what "define their function as a stand alone function and not link them"?

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Posted by ATLANTIC CENTRAL on Saturday, December 9, 2023 2:07 PM

Never mind.

    

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Posted by wjstix on Saturday, December 9, 2023 3:07 PM

I believe - unless reproducing prototype signalling is really important to you - that having signals that are as simple as possible to get the job done is a good idea. My situation was different from the OP's; although my layout covers a lot of space, it's basically an around-the-walls shelf layout and only sees one train at a time, so block signalling wasn't really necessary. However, to see the positioning of turnouts 10-15 feet away, I found using two-head interlocking signals helped a lot - especially since I want to be able to run the layout at night. I connected the mainline turnout motors to decoders so I could throw them with my radio handheld controller from anywhere in the basement.

The signals I used are from China via ebay - they're pretty cheap, and come pre-wired with resistors for the LEDs. I did use a few two-light dwarf signals too.

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Posted by gregc on Saturday, December 9, 2023 5:26 PM

wjstix
to see the positioning of turnouts ... I found using two-head interlocking signals helped a lot

what signal aspects do you use that help indicate turnout position?

what makes a two headed signal an "interlocking" signal?

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Posted by ATLANTIC CENTRAL on Saturday, December 9, 2023 7:11 PM

gregc

 

 
wjstix
to see the positioning of turnouts ... I found using two-head interlocking signals helped a lot

 

what signal aspects do you use that help indicate turnout position?

what makes a two headed signal an "interlocking" signal?

 

I suspect he is doing EXACTLY what I described above.

Generally speaking signals with two or three heads are interlocking or "control point" signals.

There are other uses for signals with multiple heads, but that is more advanced prototype stuff.....

Greg, your second question shocks me in that with all the study you seem to be doing, you don't understand this concept regarding signals.

This is an over simplification, but in my view it is the best way to start understanding interlocking signals. 

EXAMPLE - You are approaching an interlocking with three possible routes.

Those routes are:

  • Continue straight on the mainline.
  • Go thru a crossover to a parallel track.
  • leave the mainline and enter a branchline or yard trackage.

You have a signal above or to your right with three heads.

The top head has three aspects - G/Y/R 

The middle head has three aspects - G/Y/R

The lower head has two aspects - Y/R

In simple terms, the top shows the status of the straight thru route, the middle head shows the status of the crossover route, and the lower head shows the status of the branchline/yard route.

That status includes turnout position, occupancy ahead, and in CTC territory, permission from the dispatcher.

Assuming no trains in the next two blocks ahead, and no CTC, these aspects will indicate which route is selected.

G over R over R = all turnouts aligned to allow your train to proceed straight thru on the mainline, all clear.

R over G over R = all turnouts aligned to allow your train to take the crossover to the parallel track, and implies whatever speed restriction is in the rule book for that interlocking.

R over R over Y = all turnouts set for your train to take the diverging route into the branchline/yard at the necessary restricted speed.

Detection of trains on the trackage ahead will override these aspects and make them more restrictive, changing them to R or Y.

R over R over R = absolute stop.    

So even without detection, a modeler can use them to indicate turnout position and give some appearance of prototype signals.

What did you think that signals with two or more heads did?

Some railroads also used multi head signals as advance approach indications to provide status farther ahead than the next two blocks - but this is/was not common.

In between control points you would typically have automatic permissive block signals to regulate speed of trains following each other. They are typically a single three aspect head.

Looking at those rule charts for the signal aspects will never explain how they are applied to actual trackage.

You mentioned route signaling before - what I just described is route signaling at a control point. Speed signaling is exactly the same, with the possiblity that those three heads control more than three possible routes. So the Engineer does not know for sure which diveging route he will be taking, the combined aspects of the heads convey the necessary speed restrictions to him.

Example - G/R/R could be straight thru on the main, or there could be a full speed rated diverging mainline route in the interlocking. 

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Posted by gregc on Saturday, December 9, 2023 7:59 PM

ATLANTIC CENTRAL
Greg, your second question shocks me in that with all the study you seem to be doing, you don't understand this concept regarding signals.

sorry if i wasn't clear.   i was trying to ask if there's a difference between a two-headed signal and a two-headed interlocking signal?

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Posted by ATLANTIC CENTRAL on Saturday, December 9, 2023 9:13 PM

gregc

 

 
ATLANTIC CENTRAL
Greg, your second question shocks me in that with all the study you seem to be doing, you don't understand this concept regarding signals.

 

sorry if i wasn't clear.   i was trying to ask if there's a difference between a two-headed signal and a two-headed interlocking signal?

 

Well that depends.

A permissive automatic block signal will generally have a number plate, or the designation "I". Absolute signals, which include interlocking (control point) signals will have no number plate or will be identified "A".

The problem here is there are endless prototype variations on various roads at various times in history.

Other than that, simply knowing where you are on the railroad is an important indicator.

As I have explained many times, it is the opinion of some with considerable experiance that model layouts don't really need permissive automatic block signals - our distances don't really support their function in most cases. 

So a good approach is to use mostly or solely absolute signals on model train layouts - so nearly all the siginals will be interlocking signals.

Pretty simple idea, you come to a turnout or group of turnouts, two routes, two heads. Three or more routes, three heads and possibly some additional markers, dwarf signals, etc.

And again, interlocking signals do all of the jobs:

  • indicate the route, or the correct speed for a group of possible routes
  • grant permission in the presence of CTC
  • protect trains from entering occupied blocks

So why not use them to indicate route selection on our model layouts? With or without detection and other signaling features.

I'm not a western road guy, so someone can correct me if I'm wrong, but I believe the SP used two head permissive block siginals to convey the status of the next four blocks in front of the train in some locations.

So G/G meant you had four open blocks ahead. G/Y meant you only had three open blocks, G/R only two open blocks, etc. 

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Posted by gregc on Sunday, December 10, 2023 6:23 AM

ATLANTIC CENTRAL
indicate the route, or the correct speed for a group of possible routes

it's not clear to me how signals indicate route.   at least on one listing of signals (NORAC ed.7 Solomon) i see the following

  • G/R/R (green over red over red) Clear
  • R/G/R or R/G Medium Clear
  • R/R/G Slow Clear

this site, General Code of Operating Rules has

  • R/G Diverging Clear
  • R/Y Diverging Approach

can signals have different meanings depending on location?  or would operator understand the implied meaning of a signal depending on location?

ATLANTIC CENTRAL
So why not use them to indicate route selection on our model layouts?

because we don't want the expense of multi-head signals

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Posted by ATLANTIC CENTRAL on Sunday, December 10, 2023 8:16 AM

gregc

 

 
ATLANTIC CENTRAL
indicate the route, or the correct speed for a group of possible routes

 

it's not clear to me how signals indicate route.   at least on one listing of signals (NORAC ed.7 Solomon) i see the following

  • G/R/R (green over red over red) Clear
  • R/G/R or R/G Medium Clear
  • R/R/G Slow Clear

this site, General Code of Operating Rules has

  • R/G Diverging Clear
  • R/Y Diverging Approach

can signals have different meanings depending on location?  or would operator understand the implied meaning of a signal depending on location?

 

 
ATLANTIC CENTRAL
So why not use them to indicate route selection on our model layouts?

 

because we don't want the expense of multi-head signals

 

Stop looking at those charts and find a more complete book on signals.

At an interlocking that is what "Medium Clear" means, you are set for one or more higher speed dirverging routes.

R/R/G or R/R/Y means you are set for a slower speed more restricted diverging route.

G/R or G/R/R means you are clear for the primary high speed route.

So, common sense says that if there are only three routes, and there are three heads, each route is indicated by the conditions described above unless there is a train ahead that makes the signal go more restrictive.

If the interlocking is set for the mainline, but there is a train ahead of you in the next block, you will see R/R/R until it clears the block, then it will go Y/R/R when the train ahead clears that block and then G/R/R when it clears the next block.

The lower signals will stay red because those routes are not selected.

General rule books are just a guideline for developing a rule book for a specific part of a specific railroad.

And I can't say enough, before the 1980's there was no effort to make this stuff uniform from one railroad to the next.

Sheldon 

    

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Posted by gregc on Sunday, December 10, 2023 9:02 AM

ATLANTIC CENTRAL
Stop looking at those charts and find a more complete book on signals.

which one are you looking at?

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Posted by ATLANTIC CENTRAL on Sunday, December 10, 2023 9:04 AM

Greg,

Signal systems have to be adapted to endless possible track arrangements.

BUT, at its most basic level, at an interlocking, the top head indicates the status of the mainline or highest speed route.

The second head indicates the status of the next highest speed/importance route, the third head indicates the lowest, most speed restricted route - even if the routes are not phyically in that order.

The branch to the yard may be the first route you physically come to, but it will be indicated by the bottom head.

So, a single turnout interlocking would have a two head signal as you approach the points. If the top head is green, the turnout is set for the main and the track ahead is clear. The bottom head WILL be red in this case, there is no other choice.

If the turnout is set for the diverging route, the top head WILL be red, and the bottom head will be red, green or yellow depending on the speed restriction and track status ahead.

So, if you approach that turnout and see R/R, you do not know which route is set, but you don't need to know that, you know that you have to stop. But when the signal changes, then you will know which route is selected. If the top head goes green or yellow, the mainline route is set, including the turnout.

If the bottom head goes green or yellow, the diverging route is set, including the turnout.

You can believe me or not, I don't have 100 pages of prototype "proof" at my fingertips to direct you to. And I don't have time to dig thru my library, search 70 years of MR for articles, etc, to give you references to look up. 

Sheldon

 

    

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Posted by ATLANTIC CENTRAL on Sunday, December 10, 2023 9:14 AM

gregc

 

 
ATLANTIC CENTRAL
Stop looking at those charts and find a more complete book on signals.

 

which one are you looking at?

 

I'm not looking at any of them, there is more to signaling than those charts. I learned all this four decades ago, I can get by without the chart.

In real life the railroad have reasons why they use the charts and teach the engineers from a perspective that is different from how the signals work on a mechanical level - mainly that not all engineers are mechanically minded people.

But if you can find some old relay diagrams of how the signals are wired, you will see that what I am telling you is correct:

Top head = mainline

2nd head = primary diverging route or routes

Bottom head = secondary diverging route or routes

All conditions must be met to get a clear indication - one of those conditions is turnout position.

Only one route can show clear at a time, so I guess that means the turnout is set to that route if the signal is anything but red.

Sheldon 

    

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Posted by ATLANTIC CENTRAL on Sunday, December 10, 2023 9:17 AM

gregc

 

 
ATLANTIC CENTRAL
Stop looking at those charts and find a more complete book on signals.

 

which one are you looking at?

 

Do you mean what book? Try this one.

https://www.amazon.com/All-About-Signals-John-Armstrong/dp/0890245029

It was written 67 years ago.

    

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Posted by ATLANTIC CENTRAL on Sunday, December 10, 2023 9:35 AM

You want to get way into the details, find a copy of "Railroad Operation and Railway Signaling" by Edmund J Phillips Jr, copyright 1942, reprinted 1953 by Simon-Boardman Books.

or,

"Guide to Signals & Interlockings" by Dave Abeles, 2021, Kalmbach Media

or

"Operation Handbook for Model Railroads" by Paul Mallery, 1979 by Tab Books, reprinted 1990 by Carstens.

or

"Model Railroad Electrinics" by James Kyle, 1977 by Tab Books.

or 

Find Bruce Chubb's orginal relay based signal system articles in MR, sometime in the early 70's.

You will then spend hours and hours boiling down that information to what I have already explained three or four times now.

Sheldon

    

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Posted by wjstix on Sunday, December 10, 2023 2:35 PM

Sheldon has explained this very well! I'd just add that a few railroads (New York Central) used two-head signals as block signals because with the combination they had of high speeds and relatively short blocks in some areas, a train getting a yellow (slow prepare to stop at next signal) might have less than a minute to slow down to be able to stop. I think (going by memory here) green-over-green was clear, then when a train passed it went red-red, then yellow-red, then yellow-yellow, then green-green.

On my layout, I get by with two-head/two-light interlockings for the most part. If you're on the main and come to a switch that could send you to a branch line, green-over-red means you're continuing on the main. Red-over-green means you're going on the branchline. I haven't set up detection yet, but red-over-red would mean the block ahead was occupied and you had to stop.

BTW a single red signal in real railroading can often be 'permissive', you really just have to slow way down or stop and then proceed. Two head signals can be used to ensure the crew of a train knows that it's an absolute stop, red-over-red. In some cases, like where a railroad crosses another railroad, one head might be a 'dummy' head that only can show red. So green-red, you're OK, red-red, you must stop.

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Posted by cv_acr on Wednesday, December 13, 2023 2:51 PM

gregc
it's not clear to me how signals indicate route.

 

Any indication that has "Diverging" or a speed indicated in it, implies a different route being taken.

"Diverging Clear", "Slow Clear," "Medium Clear" are all "reduced speed through (diverging route of) turnouts and interlocking" and clear to next signal.

"Diverging/Slow/Medium Approach" are all "reduced speed through turnouts/interlocking at next signal.

"Approach Diverging/Slow/Medium" will generally imply that the next signal is showing a reduced speed (because of a divering route lined)

gregc
can signals have different meanings depending on location?  or would operator understand the implied meaning of a signal depending on location?

No not really. The meaning of the signals is laid out unambiguously in the rule book. However the possible indications and the reason a specific indication is displayed on a signal depend on the possible routes and conditions at the location of the signal.

But different railroads have different rule books and different sets of signal indications. You'll note that the NORAC and GCOR rule books you mentioned are completely different.

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Posted by wjstix on Wednesday, December 13, 2023 4:09 PM

gregc
 i was trying to ask if there's a difference between a two-headed signal and a two-headed interlocking signal?

Yes, but it isn't in the signal, it's in how the railroad uses signals. 

In block signaling, one head displaying two (red or green) or three indications (red or yellow or green) usually is enough. Red means the next block is occupied, yellow means the next block is clear but the one after that is occupied, and green means the next two blocks are clear.

Interlocking signals are used at places where railroad lines diverge or cross. I believe the term came about because in the days of mechanical linkages and levers being used to throw the switches and signals, the linkages were 'interlocked' in such a way that a towerman couldn't set the signals in a way that could cause an accident. Like say where two railroads crossed each other at grade; setting the signals to clear for one line automatically changed signals for the other line to stop. There would be no physical way to change the one line to clear while the other one was at clear.

Interlocking signals virtually always use two or three heads. This is partly so that the signals are recognized as being 'absolute'...a red signal doesn't always mean 'stop' on a real railroad, but if there are multiple heads (2 or 3) all showing red, it always means stop. When two railroads cross each other, you want to be sure one trains stops to let the other go by!

Interlocking also tells the crew of a train how a switch or switches are set. Green-over-red means you're clear to continue on the track you're on. Red-over-green means you're diverging off that line - onto a branchline, onto the other line of a double-track main, or onto a passing siding.

In more recent decades, railroads have switched to signals based pretty much only on speed. They often use multi-head and blinking signals to convey speed settings. So although a two-head signal in that situation would physically look just like a two-head interlocking signal, the information each indication gives would be different.

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Posted by gmpullman on Wednesday, December 13, 2023 5:52 PM

wjstix
I believe the term came about because in the days of mechanical linkages and levers being used to throw the switches and signals, the linkages were 'interlocked' in such a way that a towerman couldn't set the signals in a way that could cause an accident.

Here's one look at how the levers are connected (interlocked) with movable tappet bars and locking bars so that a proper route can be established and as you point out an erroneous signal cannot be displayed.

 Interlocking Machine by Edmund, on Flickr

I think the New York Central "Signal" film has a segment explaining and showing the levers moving. You can find it on YouTube.

This should begin at the 'interlocking' segment:

 

Regards, Ed

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Posted by ATLANTIC CENTRAL on Wednesday, December 13, 2023 7:41 PM

cv_acr

 

 
gregc
it's not clear to me how signals indicate route.

 

 

Any indication that has "Diverging" or a speed indicated in it, implies a different route being taken.

"Diverging Clear", "Slow Clear," "Medium Clear" are all "reduced speed through (diverging route of) turnouts and interlocking" and clear to next signal.

"Diverging/Slow/Medium Approach" are all "reduced speed through turnouts/interlocking at next signal.

"Approach Diverging/Slow/Medium" will generally imply that the next signal is showing a reduced speed (because of a divering route lined)

 

 
gregc
can signals have different meanings depending on location?  or would operator understand the implied meaning of a signal depending on location?

 

No not really. The meaning of the signals is laid out unambiguously in the rule book. However the possible indications and the reason a specific indication is displayed on a signal depend on the possible routes and conditions at the location of the signal.

But different railroads have different rule books and different sets of signal indications. You'll note that the NORAC and GCOR rule books you mentioned are completely different.

 

Great explaination Chris, especially this part:

The meaning of the signals is laid out unambiguously in the rule book. However the possible indications and the reason a specific indication is displayed on a signal depend on the possible routes and conditions at the location of the signal.

Sheldon

    

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Posted by wjstix on Thursday, December 14, 2023 2:16 PM

BTW if you go about 5 minutes into the NYC film, it explains their two-head (in this case, semaphores) block signaling, where having two heads allows for four indications instead of the usual three for block signaling.

Stix

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