msb@lsuc.UUCP (Mark Brader) (01/04/86)
I enjoyed the item by Bernard S. Greenberg about the NYC signalling, and I thought I'd describe the system in use here; it's similar, but not *quite* the same. As in NYC the signals can be either one-head or two-head, and under a two-head signal there may be an additional yellow light. But then, under any of the above, there may be a white light; this is used for speed restrictions. On the one-head, green, yellow, and red have the usual meanings. On the two-head, they are rendered as green over green, yellow over green, and red over red respectively. So far we are the same as NYC, except that (as far as I know) single and double red are the same here. Under a double red there can be a FLASHING yellow if the signal is so equipped. This is "calling-on" and means that there is a red ahead and it is closer than usual to this signal. Divergences are indicated by green over yellow or yellow over yellow. Until today I would have said that these indicated divergence to the right and to the left respectively, but I was just on a train that was diverted to the extra track in Davisville station to have its last two cars removed (as usual at end of rush hour), and this diversion to the right was signalled with a double yellow. So now I'm confused. It is NOT the case that double yellow is a divergence with caution, though; any divergence is treated as a caution until the next signal is seen. Now the interesting part, speed restrictions. At the entrance to any speed-restricted curve, junction, or downgrade, the first signal is equipped with a white light, but no green in the upper (or only) head. It shows stop or caution as usual if there is a train ahead, conflicting route setting, or the like. But if not, it shows caution (yellow, or yellow over green) plus the white light. (Exception: if there is a divergence at this point, it shows the divergence signal plus the white.) Passing this signal starts a timer which affects each subsequent signal in the restricted section. Initially, each intermediate signal in the restricted section displays a single or double red plus the white, while the signal at the end of the section displays a single or double red. (This assumes that the route is clear; the intermediates could be showing caution or stop.) If the train is going at the right speed, each signal will change just before the train reaches it. The red-and-whites will become caution-and-whites (signifying that they are now the beginning of what is left of the restricted section), and the final red will become whatever is appropriate -- we only know it won't stay red, because then the red-and-white before it would have been a caution instead. The red-and-white signals are only guidelines, but the final red, like all reds, has a tripper set, and will stop the train if it is passed before it changes. (Notice that this means that a train is free to traverse a restricted section at full speed -- it will be stopped automatically at the END of the restriction. Actually, a few of the longest restrictions are divided into subsections, with the final red changing to caution-and- yellow, but this is rare.) I think the white light system is poor human engineering. Trip after trip, a driver gets used to the fact that they will see a yellow-over-white when starting out from Royal York station eastbound; this is the top of a steep but mostly straight descent, which they will coast down after getting up a little speed; then the next signal will be the first of a series of red- and-whites. Since the white is at the bottom, the driver sees it from a sharp angle when the train is stopped close to the signal, and it doesn't look so bright, and they get used to that. So one time when there is a train stopped in the tunnel ahead, they see a yellow instead of a yellow-and-white, and a red instead of a red-and- white... and maybe they DON'T NOTICE, and run past that red at the usual speed limit for the downgrade, which is about the normal full speed. Now, I realize that this scenario requires the driver to be a bit inattentive, and in fact the TTC has a magnificent safety record, but that particular choice of aspects and the curious white light system does seem to make it easier for such an error to occur. And in fact I HAVE been on a train that overran a red, at speed, at that particular location. (The tripper stopped us before any collision, of course.) Over to you, Bernard. Does NYC use this method? Does anywhere else? Now a few words about a couple of other systems. Toronto, like NYC, uses 3-aspect signals. But the caution signal is almost a luxury; the braking distances are such that drivers can practically wait until they pass it before they start braking. Now, on the underground of Sydney (yes, Australia), I observed 5-aspect signals! Their speeds are comparable to ours, at least in the tunnels (which, by the way, are tall enough for double-deck trains with overhead electric power -- something also seen on the RER lines in Paris)? London goes the other way; most of their route mileage has 2-aspect signalling, just red and green. They have a form of the "creeping past red" that Bernard described. It is called the "stop and proceed rule". If a train has been standing at a red for 1 minute, the driver is allowed to creep the train past the red. This trips the tripper, or automatic train stop as they say, and the driver has to get out and reset it, which is presumed to take another minute. After this the driver is allowed to creep the train forward until the next train is in sight! Naturally this means that there is NO protection from the driver's misjudging the speed and colliding with the train ahead, and in fact this has happened a number of times, sometimes with serious loss of life. One of these days they will probably abolish the rule, but this may have to wait until they can get financing to go to 3-aspect signalling.... Most of this material is from personal observation, some from reading and conversations. I have no connection with any transit system and will be happy to have any errors corrected. Mark Brader, Toronto... ihnp4!utzoo!lsuc!msb From ARPA, append @seismo.ARPA (I think)