Diverging Route

It would be used for an indication to a diverging route

Signals

Jas,

The photo below shows the standard aspects and indications used by the C&O, Erie, NKP, and Pere Marquette. 

While the date on this diagram is 1944, I don't believe the C&O began to use three headed signals until the late 1950's.

Here's a link to Mike Burgett's Control Train Components website:  http://www.ctcparts.com/about.htm. It has an excellent explanation of aspects and indications in addition to great information on signaling in general. Mike is a professional railroader who works in the signal department of the CN. And he's also an incredible modeler. Here's a link to the website for his Clifton Forge Division of the C&O:  http://www.cliftonforgediv.com/index.html.

Tom Patterson

Signal

Home signal of an interlocking.

3 vs. 2

Specifically, 3 heads vs. 2 heads at an interlocking normally allow for higher speed indications. (for example, on Canadian Railways, having the middle head allows for "Medium" speed indications instead of only "Slow" for diverging routes. Top head green (over any number of reds) is always clear (straight) now matter how many other heads below it.

Refer to signal charts or rule books for your railroad of choice, as there are a lot of different signalling systems, and the particular aspects can vary between different railroads.

Thanks guys...

For the information, signalling can be a nightmare to research sometimes with vast variations of systems.

Tom thanks for the chart and the chat last night, and for helping out in the future greatly appreciated.

Jas...

Three heads would allow for

Three heads would allow for multiple routes.... generally speaking the lower the color (other than red) the more restrictive the route. Hope that makes sense.

Thanks Travis...

That's what I was thinking but was unsure, someone else has confirmed that some RR's used these to denote multiple routes through a plant.

Jas...

That's what I was thinking

Also watch the era.  Early signaling systems (pre-WW1) tended to use each signal for a route, but later signal rules (1930's and later) tended not to.

Multiple Routes

There could be 5 different routes past a signal through an interlocking, but if they're all the same speed, they'd all have the same indication. 

Perhaps there were some other early system pre-WWI (you're talking over a century ago here though) like Dave H. said, but any modern signalling system the number of heads has absolutely nothing to do with the number of routes, only the possible speeds.

I can give you a few hundred examples of triple headed signals where there is only a single possible route past that signal.

Chris what you're saying is

Chris what you're saying is correct, but only for speed signaling, not route based. There may be many places with only one possible route, but you most likely don't get all of the signals used in that case.

My take

I guess I will chime in and give my thoughts.

Here is a link to the signal info I used.

http://signals.jovet.net/rules/UPRR%20Signal%20Rules.pdf

This is the current Union Pacific signal rules. In my time railfanning and learning their operations, and studying how their signals operate, I have come to the conclusion that they have a hybrid Route and Speed method.

For instance, looking on that link, a Diverging Clear just says to proceed on the diverging route at the prescribed speed through the turnout. If it is a 60 mph turnout, and the maximum authorized speed for that section of main line is 60, and the maximum speed for that train is 60, then they are going to do 60 mph on either the diverging or normal route.

But a Diverging Clear Limited restricts the speed to 40 mph. So some just deal directly with routing, some just speed, and some both.

This is where an Employee Timetable would be used to list turnout speeds at each Control Point, and part of the qualification training for train crews would instructed them on specific areas prone to confusion. Look at signals from a engineers point of view. He is operating a heavy train, and needs as much pertinent information to give him enough advanced warning of any condition that would affect the safe operation of his train. Look at the signal definitions included in JMRI, to see an assortment of rules. Maybe find one that is acceptable for your layout. Just remember, this is model railroading, and not to over complicate things. Sometimes simple is better!

Steven