Application of railway signals

Transcription

In the previous videos, we introduced you to the basic, 3 head railroad signal and the three speeds it indicates: High speed, which is maximum track speed limited by the type of track and its geometries, which for some trains in Canada can be as high as 100 miles an hour. Then medium speed which is 30 mph, and slow speed which is 15 mph. Now, most switches can only be traversed at 15 mph - they are quite sharp curves. Okay - so time is money. The railroads want their trains going as fast as possible, as much as possible. Slowing down your multi million dollar train to 15 mph just to go through a switch is annoying - the train waiting for you has to wait while you sloooowly get your entire train - which could be two miles long - through the switch. At 15 miles an hour! So the railroads designed high speed switches which have very long, gentle curves, enabling the train to transit the switch going 25 miles an hour. This speed is given its own name - "diverging speed." That's faster than slow speed, but track speed might be, say, 75 miles an hour. So how do you tell the train operators to slow down to 25 miles an hour through the switch? They did this with a very cheap, very simple solution which simply modified the signals which have already existed for some 100 years: You can add a sign to a slow signal. By putting a DV plate on a signal, you upgrade that signal's slow indication to a diverging speed indication. Notice for instance that the slow to clear signal and diverging to clear signal are identical - it's just a DV plate put on the signal. The diverging to stop signal is identical to the slow to stop signal, but the slow to stop has been upgraded to a diverging to stop signal. Again, notice the fail-safe incorporated into the system: if the DV plate gets covered over with snow, or some foamer steals it, the signal reverts to the 15 mile per hour slow speed instead of 25 mile per hour. So the train would wind up transiting the switch at 10 mile an hour slower than the speed the switch was designed for. But let's up the speed a little more: let's make switches with really long curves and turnouts, designed to be taken at 45 miles an hour. Again, this is new speed with its own new name: Limited speed. So you might be able to guess what the designers did to show the train operators they can take the switch at 45 mile an hour. That's right - they upgrade a pre-existing signal. There's really only one speed they could upgrade, and that would be medium speed. So if we take the medium head and flash it green, that is an "upgraded" medium speed signal - it has been upgraded to limited speed. If the flasher fails, it just means the train will wind up going through the switch at a slower, safer 30 miles an hour instead of 45. Another way to upgrade the medium signal is to put a sign under the lights - an L plate. If the L plate happens to fall off, or gets covered in snow so you don't see it, no big deal - it's a fail safe system: the train will simply go through the switch at 30 miles an hour instead of 45. So if we have a solid medium speed green light it means medium to clear. If it's flashing or has an L plate and it's green it means limited to clear - you can cruise through the switch at limited speed - 45 miles an hour, and once your entire train is through that switch, you know you can crank it up to track speed even if you can't see the next signal, because you were just told what the next signal will be - a clear signal. The limited signals are read exactly as the medium signals are, but the speed is upgraded to limited speed instead of medium speed. A medium to stop signal is upgraded to a limited to stop signal. A medium to clear signal is upgraded to a limited to clear signal. One quick note before I move on: Notice that the plate add-ons have distinctive shapes. This is to aid in identification at a distance. The DV plate is rectangular. The R plate is square. The A plate is round. The L plate is triangular. Alright - so we've gone through these basics of the speeds and how they're shown on the signals: slow speed, diverging speed, medium speed, limited speed and high or track speed. Now it's going to get a little more complicated, but you'll see the method to the madness here hopefully. Remember that these signals show us not just what to do HERE and NOW, but also what we're going to do as much as two signals in advance. This applies to ALL of the various speeds. If we can carry on at track speed and the signal is warning us of a slower speed up ahead, then the top head will be yellow - indicating we can pass this signal at track speed, but there's a restriction up ahead. The next head down will be an indication of the speed restriction we're going to encounter. For instance, medium speed. If the medium speed is TWO signals ahead, then we flash the top head - just like before that is an "Advanced" warning, in this case, advance clear to medium. We pass by this signal at track speed, expecting a medium signal two signals ahead. Now don't forget, we can upgrade that medium signal by flashing it, or putting an L plate on it. So this would be advance clear to limited. This would be clear to limited. This would be an advance clear to slow signal. This would be a clear to slow signal. We can upgrade the slow signal to a diverging signal with a DV plate. This would be a clear to diverging signal - it's the exact same as the clear to slow signal, but the slow signal has been upgraded to diverging speed. So we come up to a switch and we're going to enter the siding. Obviously if we have to limit the speed of the train passing a signal to something slower than track speed, we're not going to use the top head. We'll make it red as a placeholder, indicating we cannot go track speed. We'll make use of the lower two heads to indicate what's going to happen now, and at the next signal. But let's say we're going to go right through the entire siding, which has medium speed switches at both ends. The opposing train is sitting there on the main track, and we're going to go around him using the siding. We get up to the siding and we see this signal. The top signal is red and is just a placeholder - we cannot go track speed past this signal. The middle head is green, indicating medium speed at this signal. If the next signal was for track speed, it would just be the middle light as green, indicating medium to clear, right? But we can't go through the next switch at track speed, so again we use the next signal down to indicate what the NEXT signal will be. In this case, it is medium speed. So this signal would be medium TO medium. Pass this signal and through the switch at medium speed, approaching the next signal at medium speed. If the next switch was a slow speed switch, then the signal would look like this: Medium to slow. We are passing by a medium signal, with the next head indicating what the next signal will be, which is a slow signal. Things get a touch more complicated from here on in. Let's say that the switch we're passing through now is a slow speed switch, but the next switch is a medium speed switch? How would you indicate it? We need to show slow speed which is usually shown by the bottom head - but we need the lowest head to indicate what the NEXT signal will be. This is where things get more specific and rules-driven. We have the Canadian Rail Operating rules where the agreed upon signal configuration and meaning is laid out. What was decided was that this signal would mean slow to medium. We can't use the top head because that indicates track speed. So we use the middle head with a flashing yellow to indicate SLOW speed here and now, at this signal. The next signal is represented by the bottom head. Obviously the next signal will not be track speed, or else we'd just stick a green light on the bottom head, called it a slow to clear and go home. The next signal is not a stop signal, otherwise we'd just make the middle head yellow, call it a medium to stop signal and pass by at medium speed, stopping at the next signal. So that narrows down the meaning of the green on the bottom head: It represents what the next signal will be, and It has to be a medium speed signal. So we pass by the slow to medium signal at slow speed, and once our train has entirely gone through the switch and past the light, we can accelerate to 30 miles per hour, or medium speed, because we know that the next signal will be medium speed. You can now start to see the patterns emerge as you actually read the signals instead of just using brute force memorization: This signal is slow to limited. Pass this signal at slow speed, expecting to pass the next signal at limited speed. It's the exact same signal we just saw, but next, medium signal indication has been upgraded to limited speed. This signal is limited to limited. Pass this signal at limited speed, expecting the next signal to be limited speed. Take a guess before I tell this next one, see if you get it right. I'm guessing you probably got it: This is a Limited to slow signal. Pass this signal at limited speed, the next signal will be a slow signal. There's some minor points to be made where the signals were somewhat arbitrary. For example, clear to medium can indicate the next signal on either the middle or bottom head. But, if it uses the bottom head, it was decided to make the track speed head green. So this is a clear to medium signal. This is a clear to limited signal. You'll also remember that if it's a two-head signal, all they've done is ditch the bottom head to save on cost. So just imagine that bottom head is there, and it's a red placeholder. This is advance clear to medium. This is clear to limited. I'll also just briefly mention flashing red lights. This is something that CN does not use at all, though they are in the Canadian Rail Operating Rules. So basically a flashing red light does one of two things: If it's indicating a signal up ahead, it's warning of a RESTRICTING SIGNAL. For example: This is Clear to restricting. You can pass this signal at track speed, the next signal will be a restricting signal. This is Limited to restricting - pass this signal at limited speed, expecting the next signal to be a restricting signal. You can figure out the various combinations you can get - basically they've upgraded the placeholder to represent a restricting signal up ahead. If they are ALL red, and the bottom one is flashing, that means take or leave the siding or other track. If we're taking a switch on to a branch line, or into a siding, that signal just simply means the switch is lined for that other track, you're going to take it. CN uses the flashing arrow in advance to indicate if we're going to take the siding, and then we get our speed indication at the siding signal itself. Alright, that was a lot to take in. In the final segment we're going to cover how to read dwarf signals, which are the short or low-mast signals you mostly see in railyards. Thanks for watching.