We covered traffic sensors in a previous thread but what about train sensors? I’m talking about the sensors on the tracks that pick up an oncoming train and signal the lights to start flashing and the gates to go down. How does the sensor account for a train’s speed? Obviously a commuter train travelling at 70 mph would need to be picked up much sooner than a freight train doing 20 mph in order for motoritsts and pedestrians to safely clear the intersection.
Another ? sort of related to this: why are there so many lights on “traffic signals” for trains? Several train lines in the Chicago area have three tracks and have huge metal structures over the track with a series of 3 lights for each track. Sometimes two of them are green and one is red for a track (or other combinations), other times they’ll all be the same color. What do the different lights mean?
The sensors for trains are just a little switch on the track. When the wheel passes it, the flange of the wheel trips the switch. They don’t take speed into account, either, nor even direction: At one crossing I often got stopped at, several times I saw a train pass the trigger, the gates go back up… and then the train back up a little and trigger the gates again, even though it wasn’t in the intersection.
As for the “traffic lights”, they tell the engineer how far ahead/behind the next train is, so they can regulate their speed. I don’t know what the specific light codes signify, though.
I’m pretty sure the sensors take direction into account. Whenever a train has passed, (in my experiences anyway) the gates go back up pretty quickly. Speed has to play a factor as well. In the Chicago area, we have commuter trains barrelling through at 50+ mph one minute and then have freight trains trudging along at 10 mph waiting to pull into the railyards the next minute. It does seem like the freights take a while to reach the intersection sometimes after the gates have gone down, but wouldn’t it be more practical to have separate sensors? Or transponders maybe that set off a certain sensor so as to give the correct amount of lead time?
So I appreciate the quick reply O Moderate One, but I’m questioning the answer.
Trains are monitored by electrical contact being made through the wheels between the two rails. Electrically isolated “blocks” of track allow for detection of trains prior to arrival at intersections, also called “grade crossings”. You may have observed that you often wait quite some time for a slow train to arrive at the crossing gates. This is because the crossings are “tuned” to the maximum train speed allowed for that block of track. That way even the fastest train triggers the crossing gates in time enough to have them safely down for traffic.
As to the signals, train signals are not like street traffic control systems. While you may have one signal to control several lanes of automobile traffic, it is different for trains. Trains take longer distances to slow down or stop. Each track must have separate signals to display whether there is another train ahead, clear track or reason to slow down. This must be indicated for each and every track so that there is no confusion. The potential loss of life or property that result from train wrecks demands this.
OK about the traffic signals…the ones I’ve seen usually have 3 lights for each track, and they’re sometimes all different colors. For example one track will have 3 red lights above it, another will have two greens and a red, and the third could be one red and two greens…nine lights in all. I know each track needs it’s own signal but why 3 lights per track? Does it have something to do with the amount of distance to the next obstruction?
More likely, you are looking at a signal set that is just before a switchyard. Each track may have three branches and the lights indicate which ones are available.
A train expert will wander by here sometime soon, I’m sure.
TaDAAAAA! (unfurls cape)
BrothaTJ’s OP is concerned with two separate issues:
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Detection of a train approaching a highway crossing at grade, and
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Signal indications controlling traffic movement on the railroad.
As far as the first item is concerned, crossing lights and gates are controlled by electrical devices connected to the tracks at some distance from the crossing. The presence of a train, with its metal wheels and axles, completes a circuit across the two rails; this is interpreted by the electrical hardware as a signal to lower the gates and flash the crossing lights. Breaking of the circuit on the other side of the crossing shuts the system off.
The distance needed for activation is determined by the maximum track speed allowable for the given rail line. Thus, for a line which allows, say 79 MPH maximum, a train approaching at a lower speed will appear to activate the signals in advance of when they are needed.
Trains could use transponders to vary the time of activation according to train speed, but this would not be a fail-safe system. Neither is the system described above, but it is more positive in operation than one that would rely on transponders.
For the second issue, railway signals must, for various reasons, indicate more than simply ‘stop’ or ‘go’. Additional indications, depending on the signal system, include ‘approach slow’, ‘approach prepared to stop at next signal’, ‘prepare to take diverging route’, etc. In addition, in the USA signals are not standardized, as they were built by many different railroads, at different times and with different philosophies as to what worked best. Railroaders working a given area must learn the applicable signal indications for that zone; if they move to different zone or railroad they may have to learn a new set of indications.
For further info on signals and what their indications mean, see the following URL:
http://www.trainweb.org/railwaytechnical/US-sig.html
Cheers,
I’ve heard of a prank where kids will place a metal rod across the tracks near an intersection…indefinitely keeping traffic blocked.
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- I used to do that, whe I was 10 or 12 or so! But not quite that, exactly: you had to have a piece of mild steel, galvanized steel or aluminum wouldn’t work. You couldn’t just lay it across the tracks - you had to place it on both rails, and push it along, rubbing both rails. -And it only worked out to about an eight of a mile from the crossing, so people could see you doing it. But it did work! My friends and I happened to discover it accidentally one day, and thought it was tremendously cool. - MC
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Another reason there has to be a signal for each track is because on some railroads, some of the tracks can be used in either direction. For example, on three-track commuter lines, the center track is often used for inbound expresses in the morning and outbound ones in the evening. Or on a two-track line, you might want one train to be able to pass another, and the signals are set up to allow the passing train to safely go the “wrong way”.
As a daily commuter, the signal “trick” that I find interesting is where a commuter station is right before a major road crossing and the gates are set up to go down as the train approaches, go back up if the train stops short of the intersection, and then go back down again once the train starts moving. Of course, not every street crossing by a station has this – it seems to be popular where the station has a lot of trains stopping there and where the street crossing the line is a main thoroughfare. And it’s not just to keep the road traffic moving; it’s a safety feature because it keeps antsy drivers from going around the gates because a train is making a long station stop and then getting smashed to perdition (always wanted to say that) by another train on a different track. Which, come to think of it, is a common experience in the Chicago area, signals or no.
Never race a train to the crossing – a tie definitely counts as a loss for the motorist/pedestrian!
Rocket, thanks a ton for the info and the link. You were exactly the type of person I hoped would happen to pass through here.
The question has been answered, but let me make it clear once again that I know you need a different signal for each track. My question was why are there multiple signals for each track, which was deftly answered by Rocket.
John- The UP West line (well I still call it the Northwestern) runs right by my house, that’s the line I was referring to with the three sets of several lights for each of the three tracks. I was going to mention that station/cross street thing because that happens whenever a train stops in Glen Ellyn, but I didn’t want to gunk up that post with even more unnecessary information. It’s nice though because it a allows a few drivers to scamper over the track on Main St. or Park Blvd. before the train leaves.