What does a yellow light mean?

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Re: What does a yellow light mean?

Postby Disney Guy » Thu Jul 16, 2015 6:40 am

From observing the signals' behavior I would guess that on the Green Line, the timers for some red signals equipped with them do not start counting down until the train is almost at that signal. Or maybe there is just a plain sensor with no timer right at the signal.

Compared with on the Blue Line where a timer is usually activated long before the train reaches the signal and the operator can adjust his speed to not have to stop at a red signal.

Some timed signals will clear from red to green if the track is clear far enough ahead.

The signal outside Waban might well have a defect. It might not be getting the condition of the next block, without which information it may not show green. That is not a high priority repair. Not getting the condition of its own block and therefore not being able to upgrade from red is a high priority item.

There exist two aspect (green and red) block signal systems. Nowadays they need additional safety measures such as a repeater head underneath for the next signal downstream, namely green over red stands for "proceed prepared to stop at next signal." The Lechmere viaduct (and I think long ago the Harvard bus tunnel) has had a two aspect signal system using yellow and red.
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Re: What does a yellow light mean?

Postby bostontrainguy » Thu Jul 16, 2015 9:27 am

Disney Guy wrote:From observing the signals' behavior I would guess that on the Green Line, the timers for some red signals equipped with them do not start counting down until the train is almost at that signal. Or maybe there is just a plain sensor with no timer right at the signal.

Some timed signals will clear from red to green if the track is clear far enough ahead.


No, the timer still has to run out no matter what the following signals show.
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Re: What does a yellow light mean?

Postby bostontrainguy » Thu Jul 16, 2015 9:38 am

CRail wrote: Also, it was pointed out to me that many of the time lights, which exist to enforce speed, do not clear in the amount of time it takes to approach the signal at the appropriate speed. This means that trains have to stop at the signal despite the fact that track speed was never exceeded. This I also don't agree with, although I assume it's so that the motorman of a train will stop regardless and not assume it will clear up upon approach.


Most drivers go track speed right up to those signals then abruptly stop. You sit there for a minute and then they rapidly speed away. They don't really function as designed and, in my opinion, are more of a nuisance than a valid safety feature. They should be eliminated or at least the timers reduced to permit a smoother more comfortable ride.
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Re: What does a yellow light mean?

Postby Disney Guy » Thu Jul 16, 2015 12:09 pm

A timing signal does not perform a true timing function unless the timer is triggered somewhat upstream of the signal in question. Then the timed interval will correspond to a distance to be traversed.

For the Blue Line, the timed distance is from the signal showing yellow and white (or a little before) down to the red signal following.

If the sensor to start the timer is at a signal resting in red, then there is the incentive for the operator to proceed as fast as is practical up to that signal. Here the timed interval will only correspond to a wait time at the signal.
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Re: What does a yellow light mean?

Postby Yellowspoon » Thu Jul 16, 2015 7:46 pm

CRail wrote: ... Observance of signals is not a topic for discussion here and I doubt you have an accurate count of how many violations might occur per annum. ...
You are correct in that I do not know the total number of violations that occur each year. I was merely passing along my personal observation that I witness one or two red light violations a year on the green line.

As for the yellow light at Waban, the question has not been answered. Let me ask it a different way: Normally, a yellow light means that the next signal is red. That's not the case here. Why won't they allow that signal to turn green under any circumstances?
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Re: What does a yellow light mean?

Postby CRail » Fri Jul 17, 2015 12:22 am

Answer:
CRail wrote: the motorman is to approach the next signal assuming that it is going to be red.
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Re: What does a yellow light mean?

Postby Disney Guy » Fri Jul 17, 2015 5:43 am

In a standard 3 aspect block signal system, yellow should be shown when the block about to be entered (home block) is clear and the next block (distant block) is is not known to be clear.

Only a T official can confirm whether the yellow signal past Waban outbound was intentionally set that way for speed control purposes, possibly relating to a track defect. As was suggested earlier, as soon as the operator sees that the next signal is yellow (or green) he might speed up again.

In an earlier lifetime I was on a 3 car PCC train heading from Boylston to Arlington outbound when the poles of the first two cars went off. A few weeks later I noticed that one of the signals in that stretch was set to rest in red (similar to Kenmore inbound to Hynes) which I suppose was for and administrative forced slowdown. The latter condition persisted for years. When and whether an overhead repair was made I do not know.
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Re: What does a yellow light mean?

Postby Yellowspoon » Fri Jul 31, 2015 3:26 pm

CRail wrote:Answer:
CRail wrote: the motorman is to approach the next signal assuming that it is going to be red.
You're emphatically repeating your non-responsive answer. That does not answer my question: Why is that signal prevented from turning green?

Normally, a yellow signal means that the next signal IS red at the current time. It might not be red when the train gets there, but it is red right now. OTOH, a green signal normally indicates that the next signal is not red, nor will it be red when the train gets there. That is not the case here. Therefore, your stock answer does not apply here.
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Re: What does a yellow light mean?

Postby Gerry6309 » Fri Jul 31, 2015 6:14 pm

Yellowspoon wrote:
CRail wrote:Answer:
CRail wrote: the motorman is to approach the next signal assuming that it is going to be red.
You're emphatically repeating your non-responsive answer. That does not answer my question: Why is that signal prevented from turning green?

Normally, a yellow signal means that the next signal IS red at the current time. It might not be red when the train gets there, but it is red right now. OTOH, a green signal normally indicates that the next signal is not red, nor will it be red when the train gets there. That is not the case here. Therefore, your stock answer does not apply here.

As I stated before, in non-cab-signal territory, yellow means "Proceed, prepared to stop at next signal."

In cab signal territory, signals only display double red, yellow over red, or red over yellow. There are no green signals. Cab signals will indicate "Stop" before the red over red is reached.

Either red/yellow variant means the same thing: "Proceed at speed allowed by cab signal, or at not more than 25 mph on bypass."

Red over yellow additionally indicates that the interlocking ahead is set for the diverging route.

Yellow over red may be displayed at locations with no interlocking, in that case it means that the line is clear to the next signal. Otherwise it means that the interlocking is set for the primary route. The first variety is a leftover from the transitional days, it still shows up at certain locations on the Orange Line.

Yellow over lunar white on the Blue Line is a timed signal, approaching the next signal at the proper speed will clear it, the red signal will blink when the timer runs out, then clear to yellow over lunar, plain yellow or green.

Yellow over Yellow on the Green Line indicates "Station Occupied", "Stop and proceed at restricted speed." There is no special marker for timed signals on the Green Line and many will not clear until a full stop is made. There are occasional locations where yellow is the best possible indication, usually at stations with obstructed views. Along Commonwealth Av. there are traffic signals for streetcars. They do not indicate track occupancy.
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Re: What does a yellow light mean?

Postby MBTA3247 » Fri Jul 31, 2015 8:57 pm

Yellowspoon wrote:
CRail wrote:Answer:
CRail wrote: the motorman is to approach the next signal assuming that it is going to be red.
You're emphatically repeating your non-responsive answer. That does not answer my question: Why is that signal prevented from turning green?

Normally, a yellow signal means that the next signal IS red at the current time. It might not be red when the train gets there, but it is red right now. OTOH, a green signal normally indicates that the next signal is not red, nor will it be red when the train gets there. That is not the case here. Therefore, your stock answer does not apply here.

There is a (formerly universal) crossover just west of Waban. The signal not displaying an aspect less restrictive than yellow might be to remind operators to proceed with caution over the switches. Despite the removal of the facing-point crossover some years ago, the T never reprogrammed the signal to display a green aspect.
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Re: What does a yellow light mean?

Postby CRail » Fri Jul 31, 2015 10:35 pm

Yellowspoon wrote:You're emphatically repeating your non-responsive answer. That does not answer my question: Why is that signal prevented from turning green?

Because it is always to be assumed that the next signal is red. That's the answer, whether you like it or not, that's the answer. No matter how many times you ask the question, that's STILL the answer!
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Re: What does a yellow light mean?

Postby Gerry6309 » Sat Aug 01, 2015 9:53 am

CRail wrote:
Yellowspoon wrote:You're emphatically repeating your non-responsive answer. That does not answer my question: Why is that signal prevented from turning green?

Because it is always to be assumed that the next signal is red. That's the answer, whether you like it or not, that's the answer. No matter how many times you ask the question, that's STILL the answer!

To clarify, again, the signal is prevented from turning green because there is no relay in the case to make that happen, thus, regardless of the condition of the two blocks ahead, the signal cannot display a green aspect. A signal system is a logic equation...

These represent 3 successive green line signals, the middle one will never display an aspect more permissive than yellow.

if .not. B193 .and. .not. B191 then S191 = Green .else. if B193 .and. .not. B191 then S191 = Yellow .else. S191 = Red
if .not. B195 .and. .not. B193 then S193 = Yellow .else. if B195 .and. .not. B193 then S193 = Yellow .else. S193 = Red
if .not. B197 .and. .not. B195 then S195 = Green .else. if B197 .and. .not. B195 then S195 = Yellow .else. S195 = Red

B is Block Occupancy

S is Signal Aspect entering block
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Re: What does a yellow light mean?

Postby bostontrainguy » Sat Aug 01, 2015 8:36 pm

Yellowspoon wrote:
CRail wrote:Answer:
CRail wrote: the motorman is to approach the next signal assuming that it is going to be red.
You're emphatically repeating your non-responsive answer. That does not answer my question: Why is that signal prevented from turning green?


The answer to your question is . . .

While it doesn't actually "MEAN" anything, it indicates that there is a crossover ahead. You will find that most crossovers on the Highland Branch are protected on both sides (from both directions) by a yellow signal. You will also see that all of these operating signals (some are inoperative) are capable of turning double red when the switches are thrown.

There is a crossover just after that signal (H 64). That is why that signal's most permissive indication is yellow.
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