I know that most grade crossing signals are activated by track circuits - however I've also noticed that if a train comes close to a crossing (but does not cross) and stop, the signals go dark quickly, and then when the train starts moving they light up again. Obviously at this point the track circuit is complete - so how does that work?
Sawtooth500 I know that most grade crossing signals are activated by track circuits - however I've also noticed that if a train comes close to a crossing (but does not cross) and stop, the signals go dark quickly, and then when the train starts moving they light up again. Obviously at this point the track circuit is complete - so how does that work?
Most grade-crossing signals have two track circuits, both using a different audio overlay frequency, the approach circuit and the island circuit. The approach circuit detects approaching trains. The island circuit stretches across the crossing plus 50 feet (or sometimes a little more) on either side of the edge of the road crossing surface. Any occupancy of the island circuit will activate the crossing signals. Any end-of-occupancy of the island circuit will deactivate the crossing signals. Thus a train leaving the crossing exits the island circuit and deactivates the crossing signals.
The approach circuit leads up to the island circuit. Most modern grade-crossing signals (everything installed in the last 25 years) use a constant warning time device, also known as a grade crossing predictor, or GCP. The GCP measures the speed of the approaching train by the change the train makes in the frequency of the audio overlay frequency. When the GCP calculates that the train will reach the crossing in 25 seconds (the standard advance warning time) + 5 seconds (the standard equipment activation time) + additional seconds, if required, for vehicle clearance time on multiple track crossings and/or for traffic signal interties, then the GCP will tell the bells, lights, and gates to activate. If the train stops short of the crossing, the GCP calculates that the time before the train enters the crossing has approached infinity, and deactivates the signals. When the train starts moving again toward the crossing, the GCP recalculates and starts the signal when it calculates the train at 30+ seconds away. Thus, the crossing receives a constant warning time of 30+ seconds regardless of train speed. With a longer warning time, people tend to drive around gates, especially since they're conditioned by experience to expect a 30-second warning time. Shorter than that, people may not have adequate braking distance to stop vehicles or clear off the crossing.
Grade crossing signals in the vicinity of traffic signals are often intertied with the traffic signals, both to lock-up the traffic signal for the roadway across the crossing until the grade-crossing signal deactivates, to prevent traffic queuing at the crossing from blocking the roadway that is parallel to the railway, and to enable clear-out of vehicles that might be trapped on the crossing by a traffic signal just beyond the crossing. Intertie circuits may create some very, very long approach circuits in order to gain enough time to cycle the traffic lights, which is a serious complexity and cost item in urban areas with high roadway traffic volume streets, and closely spaced streets.
Grade crossings in yards or industrial areas with slow trains and complicated trackage are sometimes only equipped with an island circuit because it's infeasible (or very very expensive) to install approach circuits because they would be continuously activated by switching activities even if no train is approaching the crossing. If so, there will be a timetable special instruction, general order, or track bulletin instructing trains to not enter the crossing until it is observed that the signals have been fully activated, and the island circuit may be extended some distance further to enable the train to enter it and still be able to stop short of the crossing until the signal activates. Or, the crossing might have a manual remote start (push button, radio control, etc.) that must be activated before the train has permission to enter the crossing.
The island circuit consists of a constant frequency applied between insulated joints. It doesn't have, or need, a GCP. Either the circuit is occupied or it isn't, it's an either/or circuit. The approach circuit extends from the island circuit outwards to the distance necessary that a train operating at maximum authorized track speed on the subdivision will enter the approach circuit and reach the crossing 30+ seconds later. For example, on a subdivision with 79 mph maximum speeds and a 30 second warning time, the approach circuit would need to extend not less than 3,476 feet from edge of roadway. At the distant end of the approach circuit, a narrow-band shunt is installed between the rails to enable the approach circuit to form a complete electrical circuit, but not allow the wayside signal track circuit to pass. In non signaled territory the NBS isn't needed. If there are any insulated joints for the wayside signal system within the limits of the approach circuit, a second GCP will be needed at that location to look past the insulated joint, and transfer the approach information to the GCP at the crossing. If there's an end of siding within the approach circuit, you need two more GCPs, one for each track. All in with instrument house, utility power, track cable, battery charger, remote telemetry, shunts, track cable, design, testing, installation, etc., etc, a GCP costs about $250,000 each installed, which is why grade crossings can get very expensive in an urban area.
RWM
So if you're in a region of CWR then at crossings there must be joints, correct?
Insulated joints, yes. If the crossing is signaled. If not, there doesn't necessarily have to be.
But note that many grade-crossings in CWR track use a heavier rail section through the crossing to make the crossing stiffer. For example, if the line is laid with 115, there might be 136 through the crossing. In that case, there will usually be compromise joints at the crossing in addition to insulated joints.
Railway Man [snipped] All in with instrument house, utility power, track cable, battery charger, remote telemetry, shunts, track cable, design, testing, installation, etc., etc, a GCP costs about $250,000 each installed, which is why grade crossings can get very expensive in an urban area. RWM
About a year ago a local municipality obtained quotes to install new flashers and gates at 3 successive grade crossings on a 2-track main line near here that sees about 33 trains per day at speeds up to 50 MPH, and then decided to pay to have them installed at 2 - 1 that had only crossbucks and stops signs, the other had existing flashers and gates that were about 20 years old (the 3rd closing - also crossbucks and stop signs - apparently will be closed). I had an opportunity to review all 3 of the Class I railroad's cost estimates, and because they are governmental "public records" in Pennsylvania, I am at liberty to provide the following summary of them:
Total estimates: $267,000 (existing flashers and gates), $278,000, and $312,000 (the 3rd one - close to another major grade crossing).
Days to install, for a 4-man crew, construction supervision, plus 2 trucks, 1 backhoe w/ trailer, 1 pipe pusher, and supervisor's vehicle, for all days: 33 days, 33 days, and 40 days, respectively.
For the most 'typical" one which now has only crossbucks and stop signs, the breakdown of its $278,000 total estimate is:
Materials* and freight: $99,700
Labor (direct), at $1,120 per day for the 4 men x 33 days**: $37,000
Payroll Tax & Overheads: $49,000
Preliminary Engineering: $11,600
Construction Supervision: $20,700
Meals and Lodging: $17,700
Rental of Equipment: $31,400
Construction Supervisor Vehicle: $11,000
**33 days = 6.6 weeks for 4 people ! Plus a supervisor ! Does that strike anyone else as being somewhat excessive ? I looked over the circuit diagrams, and it seems to me that starting 'cold' with someone who knows how to read them, about 4 weeks = 20 days would be enough to do this, particularly since they would be 2 or 3 similar ones right in a row. That was our only substantive comment, and so the railroad agreed to use its 'best efforts' to minimize that component. Last I heard a couple weeks ago is that the project was about 90% complete at 75% spent, so it looks like it will come in at about 15% / $40,000 under budget, which correlates to about 8 days less or 25 days total = 5 weeks. But still . . . ?
*Major material items, in no particular order:
1 ea. Grade Crossing Predictor - $38,000
1 ea, Equipment Shelter, 6' x 8' - $9,400
4 ea. Narrow Band multi-freq. Shunts - $2,424
1 ea. Battery, 368 Amp-Hours, 2.25 volts - $3,565
1 ea. FCC License for monitor/ radio - $700
1 ea. Berm Wall (acct. low sloping ground at the shelter) - $5,000
Misc. Underground Boring - $5,000 (plus the crew and backhoe and "pipe pusher" for the entire 33 days above ?!?!?)
2 ea. Pre-Cast Concrete Foundations - $1,155
2 ea. Masts w/ Base - $2,070
2 ea. Lamp Assemblies - $2,424
Accessory Package w/ E-Bell - $2,700
2 ea. Gate Mechanisms - $8,100
2 ea. Gate Arms, 17 ' - 24' - $680
2 ea. Counterweight Packages - $2,350
--------------------------------------------------------------------------------------------------------------------
- Paul North.
It doesn't get any cheaper with a private contractor, Paul. Also, railroads use system averages for cost estimates. Some installations cost more, some cost less. Thus, when you buy, say, an insulated joint, you're getting the average of all the insulated joints installed last year. Frankly, your city got an average price.
So it now seems, based on some limited checking I was able to do. Thanks for the comment and 'data point' anyway. I owe you (another) one sometime. You'd probably find the 'legacy' prints from the 1940's Reading RR days of some interest . . . the estimate for the most expensive crossing had about 70 sheets of attached signal circuits and R-O-W plans from back around then attached.
P.S. - A year or two ago we had a discussion about dual lines and sets of signals at the same road crossing. Since then I've been able to get fair to good photos of 2 of them - Harrison St. in Emmaus, and Main St./ SR 63 in Lansdale, both ex-Reading RR - with trains on one or the other line, but haven't yet posted them. This seems like as good a time as any - so sometime tonight, "if the force is with me". - PDN.
In my list of the costs for a typical grade crossing signalization above, I didn't note a couple of other interesting points:
- The "Battery" above is actually 13 of them @ $275 each.
- AC Service assembled opn 30' pole, less meter base - $1,050
- 2 ea. "Gate Saver" Bracket, spring-loaded swing-away adapter - $2,400
- Many other smaller and misc. items - 1,500 ft. of cables, clips, connectors, conduit, etc. - in the $10's to $100's range.
- For all of the gate-related materials (only), the total is about $13,700, or about 14.42% of the 'before freight' material subtotal of $94,919 (Freight seems to be a flat 5.00% added)
- The total 'per day' cost for the 4-man crew plus supervisor and equipment, etc. is about $5,060.
(Direct Labor - $1,120 [$35.00 per Hr.]; Payroll Tax & Overheads - $1,490; Construction Supervision - $630; Meals and Lodging - $540 [$135 per diem]; Rental of Equipment - $950; Construction Supervision Vehicle - $330 [per day !] ).
To 'break-out' the additional costs for the 2 gates only is an exercise in allocation and speculation, but here goes: I'd be inclined to day about 1 day for each gate, or 2 days total. But as as a check, let's apply the gates' above 14.42% of the total material costs to pro-rate the 33 days of labor on that basis = 4.76 days, so say 5 days altogether. So the calculation for the gate costs would be:
$13,700 materials/ equipment
+ 5% = $685 freight
+ 5 days crew labor at $5,060 = $25,300
Total: $39,685 - say, $40,000, or about 14.3 % of the total estimated cost of the installation.
Interesting - I would have thought the gates would be a much higher proportion of the total costs, but as apportioned on that basis, they're barely more than the Grade Crossing Predictor unit by itself.
P.S. - I still want to find and post those dual grade crossing signal photos !
Yup, sad but true. One I know of that was put in about 6-8 months ago was $250,000. UGH!!!
Nance-CCABW/LEI
“Even if you are on the right track, you’ll get run over if you just sit there.” --Will Rogers
Whether you think you can, or you think you can't, you're right! --unknown
Railway Man Most grade-crossing signals have two track circuits, both using a different audio overlay frequency, the approach circuit and the island circuit. The approach circuit detects approaching trains. The island circuit stretches across the crossing plus 50 feet (or sometimes a little more) on either side of the edge of the road crossing surface. Any occupancy of the island circuit will activate the crossing signals. Any end-of-occupancy of the island circuit will deactivate the crossing signals. Thus a train leaving the crossing exits the island circuit and deactivates the crossing signals. The approach circuit leads up to the island circuit. Most modern grade-crossing signals (everything installed in the last 25 years) use a constant warning time device, also known as a grade crossing predictor, or GCP. The GCP measures the speed of the approaching train by the change the train makes in the frequency of the audio overlay frequency. When the GCP calculates that the train will reach the crossing in 25 seconds (the standard advance warning time) + 5 seconds (the standard equipment activation time) + additional seconds, if required, for vehicle clearance time on multiple track crossings and/or for traffic signal interties, then the GCP will tell the bells, lights, and gates to activate. If the train stops short of the crossing, the GCP calculates that the time before the train enters the crossing has approached infinity, and deactivates the signals. When the train starts moving again toward the crossing, the GCP recalculates and starts the signal when it calculates the train at 30+ seconds away. Thus, the crossing receives a constant warning time of 30+ seconds regardless of train speed. With a longer warning time, people tend to drive around gates, especially since they're conditioned by experience to expect a 30-second warning time. Shorter than that, people may not have adequate braking distance to stop vehicles or clear off the crossing. The island circuit consists of a constant frequency applied between insulated joints. It doesn't have, or need, a GCP. Either the circuit is occupied or it isn't, it's an either/or circuit. The approach circuit extends from the island circuit outwards to the distance necessary that a train operating at maximum authorized track speed on the subdivision will enter the approach circuit and reach the crossing 30+ seconds later. For example, on a subdivision with 79 mph maximum speeds and a 30 second warning time, the approach circuit would need to extend not less than 3,476 feet from edge of roadway. At the distant end of the approach circuit, a narrow-band shunt is installed between the rails to enable the approach circuit to form a complete electrical circuit, but not allow the wayside signal track circuit to pass. In non signaled territory the NBS isn't needed. If there are any insulated joints for the wayside signal system within the limits of the approach circuit, a second GCP will be needed at that location to look past the insulated joint, and transfer the approach information to the GCP at the crossing. If there's an end of siding within the approach circuit, you need two more GCPs, one for each track. All in with instrument house, utility power, track cable, battery charger, remote telemetry, shunts, track cable, design, testing, installation, etc., etc, a GCP costs about $250,000 each installed, which is why grade crossings can get very expensive in an urban area. RWM
Is the NBS a box roughly 1 foot square mounted about three feet above the ballast with two cables going down into the ballast?
It seems that the track circuit is a tuned circuit that the tuning is under constant change when the train is moving in the circuit?
What frequency range is the audio?
It seems in unsignaled territory where the NBS is not required then there would still need to be a short circuit (shunt) between the rails at the end of the GCP for a specific grade crossing?
richardyIs the NBS a box roughly 1 foot square mounted about three feet above the ballast with two cables going down into the ballast? It seems that the track circuit is a tuned circuit that the tuning is under constant change when the train is moving in the circuit? What frequency range is the audio? It seems in unsignaled territory where the NBS is not required then there would still need to be a short circuit (shunt) between the rails at the end of the GCP for a specific grade crossing?
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