http://mysite.du.edu/~etuttle/rail/sigs.htm#Moti
Keep it simple and you would see $$ the RR would save in the long run.
Railway Man wrote: JacK:"Left-hand" and "right-hand" refers only to double-track, current-of traffic operation, with or without Automatic Block Signals, the old rule D-251, not CTC operation. Those of us who have been around for awhile continue to call this "251 territory," though now on GCOR railroads it is 9.14 territory, Movement with the Current of Traffic. CTC multiple main track does not have handedness. Each track is bi-directionally signaled. Without digging out my BNSF timetables, I think most if not all of the former Santa Fe west of Barstow that is signaled is CTC. There is still quite a bit of right-hand 251 territory in the U.S.; the UP has some in Wyoming.I hope I have written this clearly. RWM
JacK:
"Left-hand" and "right-hand" refers only to double-track, current-of traffic operation, with or without Automatic Block Signals, the old rule D-251, not CTC operation. Those of us who have been around for awhile continue to call this "251 territory," though now on GCOR railroads it is 9.14 territory, Movement with the Current of Traffic.
CTC multiple main track does not have handedness. Each track is bi-directionally signaled. Without digging out my BNSF timetables, I think most if not all of the former Santa Fe west of Barstow that is signaled is CTC.
There is still quite a bit of right-hand 251 territory in the U.S.; the UP has some in Wyoming.
I hope I have written this clearly.
RWM
Thanks for the clarification. That'll teach me to read the entire thread.
I have only been watching trains for a few years since I retired and recovered from some serious medical problems. 98% of the time I watch from Fullerton CA, which is, I gather, sort of the "poster-boy" for flexible operation. When I ride, it is usually on Metrolink to LA on BNSF rails and a good amount of that is on the "wrong" side. In fact, I may be doing that tomorrow to see my Doc and get some treatment.
Jack
wyomingrailfan wrote:I think I was refering to the UP only.
BNSF had some lengthly stretches of left-hand operation on the transcon until a few years ago.
wyomingrailfan wrote: The only place where trains run on the left hand is on UP's ex-CNW mainlines.
The only place where trains run on the left hand is on UP's ex-CNW mainlines.
I hope you are speaking of some specific context I am unaware of, for almost every time I go train watching I see so much left-hand running it becomes routine and draws no comment. I watch mostly at Fullerton CA where three eastbound mains split into two southbound (mostly passenger, some light freight) and two eastbound (freight and passenger) mains. The three track main at the Amtrak station is numbered 1 to 3, north to south.
The Southwest Chief almost always runs eastbound on track 1 which is left-handed. That puts it on the station side of the tracks, a major convenience for passengers. I have heard of it stopping on Track 3 but I have never seen that.
Upon occasion, when lateness puts a northbound Surfliner or Metrolink train in the station at the same time as the Chief, it also runs left-handed and uses track 3. I have also seen the eastbound Chief stopped on track 1 (left), a northbound Surfliner in on track 3 (left), and a freight rumbling slowly east on track 2 between them. When the Chief left it passed the freight and then switched over to track 2 well ahead of the freight up the Santa Ana Canyon.
I have also seen a freight stopped on track 2 with the engines right at the station and containers stretching to the western horizon. As it sat idling, a southbound Surfliner stopped on track 3 (right running), then proceeded to San Diego at the junction. Then the Chief came in (left running) on track 1, picked up a bunch of passengers, and departed, still on track 1. 5 to 10 minutes later a Metrolink 91 Line train came westbound on track 2 (left running) and switched over to track 1 to stop at the station (right running). As soon as our scanners heard the Metrolink report that it was clear of the junction and in the station, we heard the dispatcher tell the freight on 2 to go on signal indication. He immediately told the dispatcher he had a green and started rolling.
We often see freights running left handed on track 1. I have even seen a local freight run (right handed) northbound from the direction of Anaheim, merge onto track 2 and then switch to run (left handed) through the station on track 3. Since they added a 3rd main (an extension of track 1) through Buena Park to the west, they run a lot of freights east on track 3 (right hand) and switch to 2 just past the station. About the only thing I haven't seen is a passenger train on track 2 in the station.
I have ridden all of the SoCal Metrolink mileage there is and there is a LOT of left hand running in the system, particularly on BNSF rails. When watching the BNSF on a laptop with ATCSmon operating trackside, one is impressed by the flexibility and ingenuity of the dispatchers. Right hand or left, they don't care, if it works they run it.
When, in a year or two, Metrolink starts running a 1/2 hour schedule all day between south Orange County and Fullerton (or LA if they're smart), things will get even more interesting.
CSSHEGEWISCH wrote: wabash1 wrote: the is no need for a standard signal system. the public does not need to know what signal mean. and since your qualified on the area you run that is all you need The point is well-made. Unlike commercial aviation, where flight crews can get re-assigned to different routes and standardization is therefore necessary, railroad operating crews tend to stay in the operating district for their entire working careers, they need to know the signal aspects on their district only and don't have to worry about the aspects on another road halfway across the country.
wabash1 wrote: the is no need for a standard signal system. the public does not need to know what signal mean. and since your qualified on the area you run that is all you need
the is no need for a standard signal system. the public does not need to know what signal mean. and since your qualified on the area you run that is all you need
The point is well-made. Unlike commercial aviation, where flight crews can get re-assigned to different routes and standardization is therefore necessary, railroad operating crews tend to stay in the operating district for their entire working careers, they need to know the signal aspects on their district only and don't have to worry about the aspects on another road halfway across the country.
Now with that said and the understanding somewhat understood. Here is the kicker, on the roads that have system wide senority if i wanted to go to florida or texas and work my last years until retirement, It is my understanding i cant just bump to that area i haft to wait for a opening and then bid in the vacancy and then qualify myself to the area that i intend to go to. on my nickle. so if im in chicago and going to texas its my dime to move and my dime to train. im thinking maybe csxengineer can chime in here on this or maybe rodney from the bnsf these gues might have the system wide senority. regardles it dont take long to learnd another area signals, learn where the positives are and the signals around the yards and the rest falls into place.
A railroad is not like an interstate highway. It does not lollygag along and turn off of its tracks to hit some automobile. Just think if highway trucks amd cars operated in a gully they could'nt turn off of, just think how much our insurance rates would go down because of fewer accidents. Railway signaling is much much different than street signals. Like i said before followed the rules on your territory or you were history. You had the employeee timetable to go by and you stayed on your territory.
zugmann wrote:It can be done, and I'm sure the railroads would love to do it. But it all comes down to money. Standardizing the signal system would take a lot. I don't even know how much - but I'm sure it would be a big number.
I agree with you! We need to standardize our signal system in America.
Railway Man wrote: Signals convey speed and route information, not just go, stop, or about-to-change-to-stop like a traffic light... the aspect you see at a fixed signal is really looking 2, 3, 4 or even 5-6 signals farther down, and in two directions, not just one, whereas traffic lights are all one-way.
Signals convey speed and route information, not just go, stop, or about-to-change-to-stop like a traffic light... the aspect you see at a fixed signal is really looking 2, 3, 4 or even 5-6 signals farther down, and in two directions, not just one, whereas traffic lights are all one-way.
Railwayman, I know of situations, albeit rare ones, in which traffic lights work in two directions. One such situation occurs frequently in Germany when road construction is being performed without a detour. One lane is under construction while the other lane is open for traffic, which is let through in alternating directions via portable traffic lights set up at each end of the construction site. And many large cities use relays to coordinate traffic lights along major thouroughfares, so that, for example, a driver driving the speed limit could anticipate green lights at every intersection.
I also think that Larry made some very interesting points in his last post on this thread.
I guess nobody can help me get infos on Canadian signaling?
n012944 wrote: Lee Koch wrote: I don't buy the exaggerated costs being estimated here either. Every Class I has CTC on their main lines. Most of them use color signal lights in some combination of green, red and yellow. Switch the oculars if you have to (20 minutes), reprogram your signaling software (expensive, but not in the billions), perhaps change a relay here and there, and adapt your rules book. Traffic lights for roads all work the same, and can be adapted to VERY different situations. It can be done, and at lower cost than has been claimed here.You left out that all your operating people will need to be retrained. A very expensive procedure. Also while you are "switching out the oculars and reprograming your signaling software" you will either need to have an absolute curfew or a signal suspension in effect on the line that is doing the change over. If the line is a busy one both options will be VERY expensive. A curfew will back up yard for miles around, and a signal suspension will really cut down on the capacity of the line. A signal suspension on a busy line will also require require the carrier to pay for the mental heath care the dispatcher will require at the end of his/her shift.
Lee Koch wrote: I don't buy the exaggerated costs being estimated here either. Every Class I has CTC on their main lines. Most of them use color signal lights in some combination of green, red and yellow. Switch the oculars if you have to (20 minutes), reprogram your signaling software (expensive, but not in the billions), perhaps change a relay here and there, and adapt your rules book. Traffic lights for roads all work the same, and can be adapted to VERY different situations. It can be done, and at lower cost than has been claimed here.
I don't buy the exaggerated costs being estimated here either. Every Class I has CTC on their main lines. Most of them use color signal lights in some combination of green, red and yellow. Switch the oculars if you have to (20 minutes), reprogram your signaling software (expensive, but not in the billions), perhaps change a relay here and there, and adapt your rules book. Traffic lights for roads all work the same, and can be adapted to VERY different situations. It can be done, and at lower cost than has been claimed here.
You left out that all your operating people will need to be retrained. A very expensive procedure. Also while you are "switching out the oculars and reprograming your signaling software" you will either need to have an absolute curfew or a signal suspension in effect on the line that is doing the change over. If the line is a busy one both options will be VERY expensive. A curfew will back up yard for miles around, and a signal suspension will really cut down on the capacity of the line. A signal suspension on a busy line will also require require the carrier to pay for the mental heath care the dispatcher will require at the end of his/her shift.
What retraining? When the railroads change signals now and even put a compleat system in all we get is a bulliten stating what the signal aspects can be where the new signals are located and when they go into service. rules are the same so nothing new just know what it can do and run your train
RWM - Absolutely no question in my mind that the example you give would be the case in many locations.
Part of the entire process is obviously going to be a cost-benefit analysis. In the case of your example it simply wasn't worth it - for now. Down the road, other changes/upgrades in the involved signal systems may make it less onerous, especially if those changes are made with an eye to including the desired changes to that particular switch (and other involved facilities) in the plan.
My reference to red-yellow-green is meant to suggest that a significant amount of rail mileage already follows one of a very few standards. Determining national standards would eventually narrow that to a minimum (to be determined and dependent on the application, ie, ATC/ABS/etc). Any new work on a line would then be done to the appropriate standard. Exceptions would be noted just as they are today, in the timetable and rulebook. Eventually all installations will be upgraded, to the new standards.
Railroads are doing signal upgrades all the time. There is a training cost inherent in that - this would be no different. In fact, the training requirements would become less and less over time, since once a T&E employee learned the new standard, they wouldn't have to learn a new signalling schema if they moved to a different line that also used the new standard. Profile and special situations, yes. General signalling, no. For that matter, depending on what the standards become, there may be no change for the employee at all.
I don't see this as an overnight thing by any means. My position would be that there should be a standard set of aspects that fit ~99% of the track mileage nationwide. That leaves the special situations, which will always be the case. It's no different on the highway. Every drive through a traffic circle? (Many of our readers will say "a WHAT?")
The fire service has the National Fire Protection Association (NFPA). Its standards don't carry the weight of law, but they are nonetheless that which the tools and procedures we use are measured by. No railroad should be bound by law or a deadline to accept and implement the new standards (which will take years to fully implement), but with a standard against which to work, they will make progress in that direction.
Larry Resident Microferroequinologist (at least at my house) Everyone goes home; Safety begins with you My Opinion. Standard Disclaimers Apply. No Expiration Date Come ride the rails with me! There's one thing about humility - the moment you think you've got it, you've lost it...
i dont know if this has been discussed here but....the former FRA would determine speed limits based on signal type and spacing and track structure and "other indicies" including local population.....this is the reason that Amtrak from Grand Rapids to Chicago is allowed only 59 MPH on CSX track in MI
Railway Man wrote: Larry, it's not even remotely that simple. Signals convey speed and route information, not just go, stop, or about-to-change-to-stop like a traffic light. Maximum speed depends upon turnout sizes, braking distances, and what lies beyond the next signal -- the aspect you see at a fixed signal is really looking 2, 3, 4 or even 5-6 signals farther down, and in two directions, not just one, whereas traffic lights are all one-way. Changing the information conveyed by the aspect may require replacing the turnout, respacing the signals, replacing the signals, rewiring the signal progressions, or all of the above. RWM
Larry, it's not even remotely that simple. Signals convey speed and route information, not just go, stop, or about-to-change-to-stop like a traffic light. Maximum speed depends upon turnout sizes, braking distances, and what lies beyond the next signal -- the aspect you see at a fixed signal is really looking 2, 3, 4 or even 5-6 signals farther down, and in two directions, not just one, whereas traffic lights are all one-way. Changing the information conveyed by the aspect may require replacing the turnout, respacing the signals, replacing the signals, rewiring the signal progressions, or all of the above.
Okay, now I think I understand why the DB has opted to install Ks signals only on new routes, with the older routes getting updated by and by. I can see that speeds, etc. could not be signaled with just 3 aspects, and a system which conveys all the information you've mentioned would be complex.
The Ks Signal has 3 lights, red, yellow and green, which can light up in various combinations. Below the actual signal is a grid of lights that form numbers. If the grid is dark and the signal aspect green, then that is a clear at the maximum speed allowed in the schedule. Green with flashing yellow tells the engineer to check the grid. If the grid shows, for example, a 7, then the engineer may proceed at 70km/h. Solid yellow means proceed, prepared to stop at next signal. Red just means stop. The only permissive signals are in and around depots and yards. Red over yellow means proceed at minimum speed within yard limits.
Yes, semaphores sure seem very straightforward compared to modern signals. But surely CN and CP, as well as the US class Is that run in Canada see all of their needs met by the system in use. Who could describe (in abreviated form) or offer a link to an explanation of Canadian signaling?
An "expensive model collector"
I think another element of this discussion is that not every signal condition will give an exact message as to the the speed the train may operate in the track governed by the signal. For example, in ABS territory a yellow aspect can specify a maximum speed, something below the top authorized speed for the block, but also can be an indication to the engineer to be prepared to stop short of the next signal.
This is where the engineer must know his territory. In some circumstances, the engineer may be able to drift through half the block at the maximum authorized "reduced" speed and then start to reduce train speed so that a stop can be made before the next signal. In other cases, say with a farily short block governed by the yellow aspect, the engineer may have to begin the braking to stop the train within a very short distance inside the block.
"We have met the enemy and he is us." Pogo Possum "We have met the anemone... and he is Russ." Bucky Katt "Prediction is very difficult, especially if it's about the future." Niels Bohr, Nobel laureate in physics
jeaton wrote:Question here. As updated lineside signals are installed-replacement of old systems, dark to signaled, new construction, etc.-is each railroad employing a system wide standard for display aspects? It seems to me that that would make some sense. Some exceptions, but most freight train crews don't don't get off their employer's rails. With that, what would be the point of nationwide uniformity? On the other hand, is there any move for uniform standards for ATC systems? Seems like that might be driven by the need for some uniformity with the hardware in locomotives. As we all know, locomotives do frequently run off the the home road.Answered above. (CBCT)
Question here. As updated lineside signals are installed-replacement of old systems, dark to signaled, new construction, etc.-is each railroad employing a system wide standard for display aspects? It seems to me that that would make some sense. Some exceptions, but most freight train crews don't don't get off their employer's rails. With that, what would be the point of nationwide uniformity?
On the other hand, is there any move for uniform standards for ATC systems? Seems like that might be driven by the need for some uniformity with the hardware in locomotives. As we all know, locomotives do frequently run off the the home road.
Answered above. (CBCT)
Yes, each railway is standardizing itself from its pre-merger partners, as much as possible. Standards evolve too, because there's no perfect system and the world keeps changing -- trains get longer, maximum tons per operative brake increases, the pressure for creating more train slots keeps getting turned up. So there's never a point where you can say, "That's it, stop progress here, make everything look like this."
Larry, it's not even remotely that simple. Signals convey speed and route information, not just go, stop, or about-to-change-to-stop like a traffic light. Maximum speed depends upon turnout sizes, braking distances, and what lies beyond the next signal -- the aspect you see at a fixed signal is really looking 2, 3, 4 or even 5-6 signals farther down, and in two directions, not just one, whereas traffic lights are all one-way. Changing the information conveyed by the aspect may require replacing the turnout, respacing the signals, replacing the signals, rewiring the signal progressions, or all of the above. I just went through an exercise of trying to upgrade an aspect at ONE turnout, where, when we peeled back the layers, we discovered we would affect six different control points on three different railroads and would have required expending $25 million, all to try to change one signal head from a red over lunar to a red over yellow. We decided to stick with the R/L aspect.
wyomingrailfan wrote: Today signals are standard, but there are two versions. One is ATC(Automatic Traffic Control), which will stop trains moving past a red, and convential CTC. The only place where trains run on the left hand is on UP's ex-CNW mainlines.In Mexico, it's all train orders and telegraph, and I think KCSM runs on CTC or ATC.
Today signals are standard, but there are two versions. One is ATC(Automatic Traffic Control), which will stop trains moving past a red, and convential CTC. The only place where trains run on the left hand is on UP's ex-CNW mainlines.
In Mexico, it's all train orders and telegraph, and I think KCSM runs on CTC or ATC.
There's only about 25 miles remaining of left hand current of traffic on the ex CNW lines. That will be gone by the end of the year or sooner. It's being (and has been) replaced by CTC where trains can run on either track in either direction.
The system that stops a train going past a red signal is Automatic Train Stop. Automatic Train Control is different. There is also ABS without CTC.
Everything is standardized, but by company. Local deviations are covered by special instructions. There is no real benefit to standardize everything across the entire country. You are either qualified to run your area or you are not. I know the signals I encounter. I don't need to know the signals that Wabash encounters on his run. The same goes for him. If one of us detoured over the other's line we would need a pilot who is qualified or become qualified ourselves to run without one.
For whatever reason, a human being slipped up. That happens. Anything a human does or makes, has the potential for not being perfect. Positive Train Control, whenever it finally gets deployed will probably lessen, but won't eliminate the potential for collisions. If anyone thinks that there can be a system that will never let a collision happen again, I have a bridge for sale. Cheep.
Jeff
I don't think anyone is going to question the red=stop, green=go concept. It is standard across the land.
Interlockings are a different animal, of course, and some are more unique than others (how do you approach a unique interlocking? You 'neak up on it...).
It's all those gray areas that seem to be the problem. Either a given aspect only exists on this fallen flag's lines, or the rules for it are different.
To try and draw slightly different angle here - what if one state used arrows to signify left turn permissions at intersections (as most do nowadays), but another simply added a lunar to the rest of the lights? If the light is green and you have a lunar as well, you're clear to make a left turn. Another state uses a flashing amber, with a lunar to indicate that you can make a left turn, but watch out for oncoming traffic.
Hopefully you get my drift.
The railroads do tend to be a lot like the fire service - only the fire service has been doing it longer - 300 years of tradition unfettered by progress.
Standardizing signal aspects across the country can hardly be a bad thing. What needs to be discerned is exactly where there are aspects that are inconsistent, why they exist, and what the best solution would be (ie, what aspect should replace the existing aspect and still accomplish the desired result).
Since so much of the country is already on at least a similar sheet of music, sorting out the "trouble" spots might actually turn out to be less painful than years of work and millions upon millions of dollars.
Those unique locations (and the track profile) notwithstanding, I'd bet that any of our resident engineers could run virtually anywhere in the country. Red=stop, Yellow=slow down, Green=go.
Lee Koch wrote:Thanks, Railwayman, I am glad to know that somebody out there does know what they're talking about. Didn't mean to ruffle anybody's feathers, though. And while I have to admire the skills of a locomotive engineer who has passed tests on varying signal systems, it still seems to me like savings would be greater in the long run. Instead of having to teach personell all the varying systems, just teach them one. That would also make it easier for somebody to change employers, and would save the new employer from having to re-train the guy. No departments having to draw up railroad-specific rules, because they'd be the same at every railroad. Uniform cab signals would also make pooling motive power easier. I just don't get why the railroads get so defensive about their own signals/rules, etc. And as to the argument about wanting to know what's going on, yeah, I am a die-hard rail fan, and one of the aspects of the hobby I really enjoy is reading signal aspects. But it's almost like some folks want signaling to be priviledged information available only to members of an elite fraternity, or something.And of course, the whole question of uniform signaling ties in with the concept of open access, but I don't think we want to go there in this thread.
Thanks, Railwayman, I am glad to know that somebody out there does know what they're talking about. Didn't mean to ruffle anybody's feathers, though. And while I have to admire the skills of a locomotive engineer who has passed tests on varying signal systems, it still seems to me like savings would be greater in the long run. Instead of having to teach personell all the varying systems, just teach them one. That would also make it easier for somebody to change employers, and would save the new employer from having to re-train the guy. No departments having to draw up railroad-specific rules, because they'd be the same at every railroad. Uniform cab signals would also make pooling motive power easier.
I just don't get why the railroads get so defensive about their own signals/rules, etc. And as to the argument about wanting to know what's going on, yeah, I am a die-hard rail fan, and one of the aspects of the hobby I really enjoy is reading signal aspects. But it's almost like some folks want signaling to be priviledged information available only to members of an elite fraternity, or something.
And of course, the whole question of uniform signaling ties in with the concept of open access, but I don't think we want to go there in this thread.
Lee: Teaching people the different systems in the operating crafts is not hard, or expensive, or time consuming -- at least when I learned them at my Class I, it didn't seem hard to me. And you get to carry a "cheat sheet" -- the employee timetables -- with you at all times. In signal engineering it gets quite a bit more complicated but I don't have any trouble shifting from one railway's system to another on a daily basis. The number of accidents caused by trainmen misinterpreting a signal is a very small number -- maybe once a year. I don't see it as a problem begging to be solved when there are so many much more important safety issues that do kill large numbers of people, e.g., grade-crossings.
We don't have too much cab signaling in the U.S. on freight railroads. Within 10 years it is expected to be gone, replaced by CBTC, on which BNSF, NS, UP and CSX have essentially agreed on a common standard.
Signal departments sometimes do come across as defensive, and not without reason. Very few people at railroads outside of the signal department understand how the signal system works and how easy it is to make mistakes. When I moved into signal from operating I have to say the learning curve was even more unpleasant than it was in operating, and it wasn't any fun at all learning operating. Signal engineering and maintenance is not rocket science but the details are all critical, each detail affects every other detail, and overlooking any one of them is disastrous.
Signaling is different between railroads because each railroad has different geography and traffic, and each railroad wants to get the most out of their property for the big money that signaling costs. Just one simple CTC siding costs at least $1.5 million for the signals alone. One-size-fits-all solutions are usually inefficient solutions. It would be cheaper to sew and sell clothing if it all came in one size, but it would fit almost no one.
Railway Man wrote: Lee: Thanks for sharing your opinions so candidly. My day job includes directing a 20-person signal design team, preparing cost estimates of signal installations, and writing railroad business plans including return-on-investment calculations for everything from new main tracks to short-line rail rehabilitations to freight opportunities large and small, so if there's anything further I can add or explain, please don't hesitate to ask.RWM
Lee: Thanks for sharing your opinions so candidly. My day job includes directing a 20-person signal design team, preparing cost estimates of signal installations, and writing railroad business plans including return-on-investment calculations for everything from new main tracks to short-line rail rehabilitations to freight opportunities large and small, so if there's anything further I can add or explain, please don't hesitate to ask.
Jack_S wrote:Let me toss in a wild card. What about signals where the colored lights compete directly with the rising or setting sun? At Fullerton CA, when the sun is setting at one of the Equinoxes (like now) it is setting right behind the signals west of the station. I have some difficulty ascertaining if the signal is flashing or not, let alone whether it is yellow or red. Green is easier to see but not by much. And I have good, often tested, color vision. (I worked in electronics, and job entry requirements often included a color vision test.)This is one area where the old semaphores had a distinct advantage.Jack
Let me toss in a wild card. What about signals where the colored lights compete directly with the rising or setting sun? At Fullerton CA, when the sun is setting at one of the Equinoxes (like now) it is setting right behind the signals west of the station. I have some difficulty ascertaining if the signal is flashing or not, let alone whether it is yellow or red. Green is easier to see but not by much. And I have good, often tested, color vision. (I worked in electronics, and job entry requirements often included a color vision test.)
This is one area where the old semaphores had a distinct advantage.
Semaphores indeed had an advantage in that situation -- plus, you can see them from the back side, too, which was nice back in the track-car lineup days.
However, (1) the engineman must know where the signals are on the territory, and (2) if the signal aspect cannot be determined, then the engineman must regard that signal as displaying its most restrictive aspect.
There are other common reasons for a signal aspect to be indeterminate, such as lamp failure, the head has turned sideways due to wind, vandalism, power or circuit failure, mast is down because it was struck by a shifted load or wayward rubber-tire vehicle, snow is blocking the lens, etc.
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