When a railroader refers to “a signal,” is he talking about the mast and everything on it, in aggregate?
Or is “a signal” the name for each appliance on the mast? So, is a mast with three semaphores on it “a mast with three signals on it.”
I ask because I’ve seen mention of a “three-blade semaphore.” And I’ve certainly never seen a single appliance with three different blades; nor can I imagine how such a beast would be used.
Is the terminology standardized?
Is a mast with three searchlight heads a signal? Or three signals?
A signal is each complete indicator for a track. If we had a overhead bridge in double track territory it would have two signals on it, one for each track.
The CROR uses both “head” and “aspect” to refer to each individual light on a signal. We don’t have any semaphores left and to my knowledge Canada never had any position light signals.
The terms used may vary between railroads and rulebooks, just like the meaning of signal indications.
For example, in the CROR a single aspect signal displaying a flashing yellow indication is “Advance Clear to Stop”, and it means that the next signal will be displaying “Clear to Stop” (solid yellow on top, with any lower aspects red) and the train must be prepared to stop at the second signal.
On BNSF this same indication is called “Approach Medium”, and it means that the train must approach the next signal at no more than 40 mph and be prepared to enter a diverging route.
When talking about signals, we usually are talking about block or interlocking signals. But MC is correct that the term “signal” encompasses more than that. Anything that conveys information affecting the movement of trains or engines is a signal.
It is the combination of colors in that stack of three (or two) that conveys the information. Books have been written on the subject, and there are variations between railroads as well. That was the cause of a rear-end collision near Chicago some years ago involving Amtrak. The engineer read the signal correctly - but for a different railroad. In that case he should have read it as a more restrictive aspect.
If I recall correctly, some railroads differentiated between heads (searchlights) in line vertically and offset (one on either side of the mast). I don’t remember what the distinction was.
B&O’s color position lights (and similar systems on PRR, N&W, etc) are a different animal, although once you get the basics down, they aren’t bad.
IIRC, the signals that had the signal heads “offset” were permissive. That is a train could pass them after stopping. Signals with the heads “in line” were absolute. A train couldn’t pass an all red signal without further authority from whomever controlled the signal.
The verticle distance between the signal heads also could change the meaning of a signal. After some of these practices fell out of favor, rule books would carry the note that the physical make up of a signal (offset/in line, etc,) did not affect the indication of the signal.
A hundred years ago, semaphore signals in the US could be upper-quadrant or lower-quadrant, which means what it sounds like: an upper-quadrant blade would be horizontal for stop and would turn 90 degrees upward to show a clear signal, and a lower-quadrant blade would be horizontal for stop and would turn downward to show clear. (Usually 60 degrees, but rarely 90 degrees.)
The difference was, upper-quad blades could have three positions – horizontal, vertical and 45 degrees. Lower-quad blades almost never did that: the blade could be horizontal, or tilted 60 degrees down. No in-between.
So how would a lower-quad signal show the equivalent of a yellow light – no need to stop, but plan to stop at the next signal? It needs two blades. That was the usual on many RRs circa 1910, before upper-quad semaphores became common. It was still the usual on SP and UP in 1940 - 1950; SP still had lots of two-blade semaphores in the early 1980s.
When you look at a pic of an SP two-blade semaphore – there are lots of pics out there – you’ll notice the two blades are almost always different, indicating their different meanings.
(In that pic, the upper blade has started to drop – he took the pic from the tail end of his train. Note milepost 589; he’s at the east end of Garnet.)
Here’s a NY Central rulebook – it shows lower-quad and upper-quad aspects starting on page 80.
In Canada the offset and single aspect signals normally cannot display “stop” (there is an exception for a single aspect signal with a letter “A” plate, though I’ve never seen one on CN). A solid red indication on these signals means “stop and proceed”, or far more commonly “restricting” if the signal has a letter “R” plate, again, I’ve never seen one of these signals without an “R” plate on my territory.
In the CTC territory I work single aspect and offset signals are always intermediate block signals that are not directly controlled by the RTC, I’ve never seen one at a controlled location. Controlled signals are always non-offset two or three aspects.
Things may be different in other areas or on other railroads.
In the old days in the US – a few decades ago, say – single-head signals often did show absolute stop. If it was red, with no numberplate, it meant stop and stay. If it was red, with a numberplate, it meant stop, then proceed at restricted speed.
Dunno how common single-head absolute signals are now, but I doubt that they’re rare in the US.
I have a failing memory. However I remember traveling west along what was the NYC track in Indianna near Chicago. At a very large lift bridge there was a signal that had 4 seperate heads. All 4 did show red. Fault memory or what ?
The railroad drawbridge along Interstate 55 in Chicago (over Bubbly Creek…you don’t want to know!) had absolute signals protecting it at either end with four heads. Somewhere I should have what those indications were. It involved the fact that three railroad lines (ATSF, GM&O, and IC) converged to cross this two-track bridge, then diverged again on the other side.
All Chicagoans know about Bubbly Creek, it had its source within the Union Stock Yards. The waste from carcasses from the various meatpacking plants was dumped into the creek. As it decomposed and decayed, gas bubbles would form and rise to the surface.
