schlimm dehusman Cab signals are for track occupancy and routing. They do not reflect temporary or permanent speed restrictions. Train ahead. Restrictive signal. Switch lined other than normal. Restrictive signal. Track clear and switches lined normal. Clear signal. In your opinion. Jeff H seems to be saying something else, and he would know. "Cab signal systems that have speed enforcement equipment will have the PCS open for overspeed conditions. Cuts power and makes a penalty brake application. If you are below the overspeed setting for a certain signal indication and then go above the setting, the overspeed kicks in."
dehusman Cab signals are for track occupancy and routing. They do not reflect temporary or permanent speed restrictions. Train ahead. Restrictive signal. Switch lined other than normal. Restrictive signal. Track clear and switches lined normal. Clear signal.
Cab signals are for track occupancy and routing. They do not reflect temporary or permanent speed restrictions.
Train ahead. Restrictive signal.
Switch lined other than normal. Restrictive signal.
Track clear and switches lined normal. Clear signal.
In your opinion. Jeff H seems to be saying something else, and he would know.
"Cab signal systems that have speed enforcement equipment will have the PCS open for overspeed conditions. Cuts power and makes a penalty brake application. If you are below the overspeed setting for a certain signal indication and then go above the setting, the overspeed kicks in."
What Dave said is just as true as what I said. Most cab signal systems only respond to track occupancy. The speed control enforcement enforces the speed associated with the cab signal indication. Unless an artificial cab signal condition is induced for a speed restriction, the cab signal won't "see" it.
Not all cab signal systems have a speed enforcement component. Those that do are usually referred to as Automatic Train Control.
Jeff
Dave makes a valid point about signal aspects. Rapid transit systems will use a distinct aspect for speed control compared to an aspect related to track occupancy. As an example, CTA used yellow-over-lunar to control speed on the grades that led into the subways as opposed to yellow to indicate approach.
Except when they do. If Amtrak wants a cab-signal restriction approaching a curve they can add it, as they did at Elizabeth 15-20? years ago.
Cab signal indications have very specific meanings and present a very limited range of options.
Displaying a signal for a condition that doesn't conform to the indication is really dangerous. If you display "approach" (which means to slow down to 30 mph and prepare to stop at the next signal) as a general "slow down" signal, then you dilute the meaning of the signal. That is hugely dangerous because it lulls crews into thinking that they got an approach because of a speed restriction when in reality they got an approach because the next signal is red.
Dave H. Painted side goes up. My website : wnbranch.com
Cab signals, as they exist today, and PTC are totally different.
Never too old to have a happy childhood!
dehusmanThey do not reflect temporary or permanent speed restrictions.
C&NW, CA&E, MILW, CGW and IC fan
Excuse me, "what happened in Philly." My understanding is that the old PRR cab-signal system with some speed restriction control, at least requiring acknowledgement if not a brake application, might have prevented the accident, but had been removed in preparation for the new system which was still to be installed and now is installed. Am I mistaken? The cab signals still worked, but the speed control had been deactivated and not yet replaced on the specific track.
gardendance Paul, nope, I didn't think our modern railroads would invest in a spped limit system that wouldn't know to turn off the motors when it applies the brakes. I'm thinking more that perhaps the locomotives are more powerful than a superman, and the sensors or time lag between when the system detects a speeding train and when the system takes action might be long enough that the locomotive could accelarate quite a bit before getting its brakes applied. Existing signal systems, either wayside or cab signals, have discrete sensing sections, don't they? Is it possible that a train could pass a sensor below the speed limit, but accelarate fast enough after to cause problems before the next signal catches it? I'm assuming Positive Train Control should not have such a limitation, that it uses some sort of radio or GPS gizmo to keep track in such short segments that it's as close to real time, and sensing the train in fractions of a second. If not, then I feel it's even more of a waste of money with little benefit.
Paul, nope, I didn't think our modern railroads would invest in a spped limit system that wouldn't know to turn off the motors when it applies the brakes. I'm thinking more that perhaps the locomotives are more powerful than a superman, and the sensors or time lag between when the system detects a speeding train and when the system takes action might be long enough that the locomotive could accelarate quite a bit before getting its brakes applied.
Existing signal systems, either wayside or cab signals, have discrete sensing sections, don't they? Is it possible that a train could pass a sensor below the speed limit, but accelarate fast enough after to cause problems before the next signal catches it?
I'm assuming Positive Train Control should not have such a limitation, that it uses some sort of radio or GPS gizmo to keep track in such short segments that it's as close to real time, and sensing the train in fractions of a second. If not, then I feel it's even more of a waste of money with little benefit.
Cab signal systems that have speed enforcement equipment will have the PCS open for overspeed conditions. Cuts power and makes a penalty brake application. If you are below the overspeed setting for a certain signal indication and then go above the setting, the overspeed kicks in.
Not quite sure what system you are referring to. PTC will apply a penalty application when the train is about to exceed the maximum authorized speed. Don't understand why you think the train will somehow keep accelerating a significant time after that.
Even with PTC is will be possible for the engineer to apply both the brakes and the power at the same time.
Most railroads have engine overspeeds that aren't tied into the track at all. The engine max speed is set. Tied in cab signals are a set max speed based on signal indication, which is not the same as track speed. The engine doesn't know the speed of the track at all and the signal system doesn't know the speed of the train. That communication is what PTC is about and why they are spending billions of dollars to establish that communication.
That is very different than what happened in Phillie. The train was probably operating on a clear signal, max speed. The cab signal system doesn't know the speed of the track. So clear could be 150 on tangent or clear could be 50 in a curve. If the train passed a restrictive signal, the train would have to comply with the speed of the restrictive signal.
People highly overrate GPS. Engines may know where they are all the time, but that doesn't mean they communicate where they are to the outside world. For example some railroads only record where the engine is every 15 minutes and then upload it to the satelites once every 2 hours.
How fast is too fast is more a function of safe track speed than of locomotive capability.
One well-known class of locomotive was known for its ability to roll a moderately heavy train at a sustained 90mph without strain. One suffered a fatal rollover when it entered a curve too fast. The curve speed was posted at 15mph.
And now you know why N&W preserved 611. After repairs, she was the best J in the fleet.
As others have indicated, it's possible for modern electronics to prevent overspeed derailments - as long as they work, and can't be overridden.
Chuck
Unless you've got one of those new-fangled A1A-A1A Diesels, all axles are driven - there are no idlers.
It is very possible for one axle to slip to the point that it causes an overspeed. I've had it happen. The power is also cut, and there is a penalty brake application, at least on our RS18u's.
Even our vintage Diesels cut power when the brakes dump.
Modern locomotives have very sophisticated wheel slip sensors - all you'll note is a "tunk" as the wheel slips and power is reduced to that axle.
As for the base question, of course. It depends on the conditions and circumstances. Even a 600 HP SW1 might be too fast to be safe...
Larry Resident Microferroequinologist (at least at my house) Everyone goes home; Safety begins with you My Opinion. Standard Disclaimers Apply. No Expiration Date Come ride the rails with me! There's one thing about humility - the moment you think you've got it, you've lost it...
GPS provides near instantenous speed measurement without measuring between defined points. (I have a GPS transponder on the data aquisition system on my race car - it provided speed and other GPS dependent data for every foot the race car travels in and around the track) IF PTC has track speed and temporary slow orders added to the GPS data base for PTC operation - PTC knows how fast the allowed track speed is.
I'd think that all a Speed Control needs to know is the speed limit on the section of track that the engine is on and the speed of the wheels. No need to rely on GPS to measure the speed between two places or some trackside sensor to tell the train how fast it is going. Granted, it might be traveling slower than the drive wheels indicate but surely there are undriven wheels that could measure the real speed of travel... it would also serve as a wheel slip sensor... If the drive wheels are going faster then the sensor wheel, then (figuratively) back-off on the gas pedal.
Semper Vaporo
Pkgs.
Patrick Boylan
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This seems kind of like asking "If I floor the gas pedal in my car and stand on the brake at the same time, what happens ?"
Does your question presume that the overspeed control would only apply the brakes, and not cut off traction power to the locomotive ?
On most diesels, when the train goes into emergency, there's an automatic cut-off of power - except when it's in dynamic braking mode, such as holding the train back on a descending grade, and then leaving the power on to continue the braking is only a good thing.
Most diesels - even on a lightweight passenger train - no, I'd think.
A high-horsepower electric with unlimited power from the catenary, substantial short-term overload rating, and acceleration capability ? Perhaps. But if the trailing cars weigh as much as the locomotive (or maybe 35%/ 25% = 1.4 times as much), their braking ability will be more than a match for the locomotive's adhesion and tractive effort. As soon as the train brakes are fully applied on 2 or 3 cars- a second or two at most for a short train, maybe another second for a longer one - the cars will start to drag on the locomotive and slow it down, causing the loco to slip its wheels, even at max. power and with super-adhesion conntrols (see the 35% above). It would be kind of like a 'tractor pull', where the sled load gets progressively harder to pull as the tractor goes farther across the stadium floor.
There will surely be a lot of flat wheels, though.
- Paul North.
Can locomotives be so peppy that any overspeed controls would render them useless?
This is kind of related to the Amtrak 188 wreck at Frankford Junction Philly. If I remember correctly the train didn't start speeding until near the end of 80mph territory going into 50mph territory and finished at over 100mph. It also seems to me that it traveled a relatively short distance to accelerate those 20 or so mph, when it should have decelerated about 30 or so mph.
I've also read some posts that said although Frankford Junction curve's speed limit is 50mph that it's possible to traverse it at 80mph without derailing.
Is the ACS64 so poweful that it could have accelerated from 50 to 80+mph so fast that an overspeed control would not have slowed it in time?
Of course I may be way off on my numbers in this case, but in any case it seems to me that it's possible to have a locomotive whose accelration is so good, and some speed limits so restrictive, that no overspeed control would be worthwhile. Also of course there are some whose opinion is that Positive Train Control's cost-benefit ratio makes it not worthwhile either.
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