Is it important for a train to accelerate through curves in mainline operations?
Im not sure what your asking but it dont matter if im up hill down hill going thru a curve i am allowed to accelerate just dont want to tear it up
Judging from this and other of your questions, you appear new to railroading. Suggest you read a few books at your library and read magazines liike Classic Trains and Trains. Directly to this and your other speed question: track speeds are set and are noted in the employees timetable instructions; usually a general speed over a certain section or division and specific speed restrictions due to curves, grades, type of engine, type of train, length of train, type of equipment, special cars, etc. are listed. As for going through a curve, its not like an automobile: a railroad speed restriciton often is for the enitre train unless otherwise noted. So a curve restricted to 45mph with 65 mph tangents on either side would usually restrict the whole train from the time and spot the locomotive enters the curve until the last car leaves the end of the curve. Get around some railfan groups, visit tourist operations, keep your eyes and ears open.
Second to Henry’s suggestions.
Further, there’s a whole lot of “it depends” there. It might be important that a train decelerate (brake) through a curve.
Depending on the curvature, it could be very important that the train neither accelerate nor decelerate, as both could cause derailments. The variables are endless.
Thanks for the replies…the question was too basic…should have asked how centrifugal forces affect train performance and handling…that’s what I was getting at. .
As normal track speeds rise, you’ll see the introduction of superelevation in curves. All the other stuff still applies.
That is an interesting statement if only because super elevation has been eliminated on many curves on many lines especially where passenger trains no longer travel. Higher and longer cars’ dynamics in today’s freight trains need a flat curve (I am told). Whether or not we will ever (re)achieve need for superelevation of curves for freight is a question I have wondered about.
Why would higher and longer cars require a flat curve? That doesn’t make sense. Centrifugal forces exist regardless of what is rounding the curve…ie…passenger or freight… Centrifugal forces act laterally on the train as it rounds the curve…superelevation simply transfers a component of that lateral force into a vertical force on the train… end result: the lateral force that is acting to push the train off the rails is diminished through curve superelevation.
I am not an engineer or track engineer…but I was told that the dynamics of the larger cars on superelevations did not track through well, something about top heavyness at the time combined with slower speeds of freight trains; super elevation was an unneeded expense. I hope some track engineer is available here to explain it in better terms.
Perhaps MC will weigh in.
I’m not a track engineer. (And I didn’t stay at “that hotel” last night either) so the following is based on my own knowledge/opinion…
The desired track speed and degree of curvature are what will determine the need and amount of superelevation. The desire is to keep the center of gravity between the rails, considering the centrifugal forces involved. I would suspect that even a freight line that runs consistantly at a higher speed will have some degree of superelevation.
On the other hand, a line that general handles slow trains will be built flat. Lines that handle both may have a compromise - enough to handle higher speeds, but not so much that tall cars are going to fall over.
Consider, too, that running higher speeds on flat track curves will create increased wear on flanges and the inside of the outside rail. A bit of superelevation will help keep the tread of the wheels on the head of the rail.
I have no idea where the division between “lower speeds” and “higher speeds” may be, other than it probably depends…
Well…if I remember my HS physics correctly trains in mainline operation would need to ACCELERATE through curves in order to generate the centripetal force required to counter that outward centrifugal force. No?
With no acceleration all you’ve got is the flanges countering that outward centrifugal force…which would be fine at slow speeds but probably not so fine when you’re running a doublestack train through a curve.
Superelevation would reduce lateral forces and consequently lesson (although not eliminate) the need for acceleration in curves.
Briefly:
No, not important to accelerate through curves. Trains have way less power per ton than a sports car (like maybe 1 / 100 as much), and so don’t accelerate that quickly. Others with more train handling experience can comment on whether and how the slack (between the couplers) should be “bunched” in, “stretched” out, or otherwise;
Curves do slow trains down a little, though. That’s why the profile grades are compensated for curvature, usually by reducing the grade by from 0.04 to 0.05 % per degree of curvature. For example, a 5-degree curve (radius = 1,146 ft., approx.) would have compensation from 0.20 to 0.25 %, which would reduce an otherwise 1.00 % profile grade to 0.80 to 0.75 % respectively (“ruling” grade is a separate discussion).
Passenger trains needed more super-elevation (“S-E”) than freights because they usually went faster (see other current thread on engineers and speeds) and for passenger comfort. But even when passenger trains went away, freights still need some S-E - just not as much, for the reasons mentioned by tree68 and below.
As tree68 also mentions with regard to rail wear and damage, S-E can be too much of a good thing. But S-E also needs to be reduced a little bit as a compromise or balance between the equilibrium S-E for the max speed, and the “worst case scenario” - that reduction is known as “imbalance” or “underbalance”. That scenario would be a heavy freight train pulling hard on a grade, and so going slow around a sharp curve with lots of S-E. Unfortunately, the slow speed doesn’t need or utilize the S-E, so this train tend to tip to the inside of the curve. Now, suppose that “worst case” train has some long, top-heavy cars like 89 ft. auto-racks (old term - now “multi-levels”) or piggyback flats with long couplers - so that each car occupies a significant fraction of t
Thank you…that’s very interesting.
One reason is to keep them from “Stringlining”. Stringlining is where the cars due to heavy forces want to tip over to the inside of a curve. If you ever played with model trains, you will know what I’m talking about.
We had one situation like this happen last year while the train was basically stopped! The train was going up a heavy grade when it went into emergency for whatever reason. All of the slack came in and bunched the train up. After the conductor walked the train and found nothing wrong, the engineer released the air brake. As the brakes released the train stretched out and several empty cars were pulled off their trucks and turning over to the inside of the curve.
Thanks Big Jim
I thought the original question was posed in relation to road vehicles for which there seems to be a theory of better tracking through a curve if the vehicle accelerates through a curve. Since the term, accelerate includes the twin characteristics of increasing tractive power and of increasing speed, I am not sure which of these would be responsible for better tracking. It seems to me that it would be only the increase of tractive power that would contribute to better tracking. I can’t see why increasing speed would help track through a curve. If anything, a curve requires a slowing of speed.
Furthermore, if there is a benefit to tracking of a road vehicle through a curve that results from increasing power, I do not understand why that is. But I have heard that if you happen to inadvertently enter a curve at too high a speed, you are better off hitting the accelerator than hitting the brake. Whatever physics principle that is behind this operating practice, I think I can detect it working while driving if I accelerate out of a curve.
Oh there are all kinds of dynamics working on a train. As I said earlier, I am niether a locomotive nor track (structural) engineer. But I do know there are rules such as when there is too much side sway in a car movement that if you cannot attain and maintain at least 25 or so mph the speed must be brought down to under 12 mph; so track, train, and speed do have impact on behavior and handling.
Similarly when going through a crossover, when does the engineer increase a speed? Supposedly when the whole train has passed through. But once rode the 13th car of 13 car train through a 20 mph crossover when the enineer apparently thought he had ten or fewer cars. So, knowing your train, knowing your track, knowing your speeds, and what all else you must know to be an engineer, all add up to the skill needed.
In auto racing there is a saying - “In slow, out fast. In fast, out backwards.”
Big Jim’s story about stringlining after an emergency stop is interesting, and believable–but would you design your railroad around unexpected emergency applications? Would it be feasible for an engineer to request permission to back out of such a curve before proceeding, if he knew that he had those stupid empty Center-beam cars together in his consist?
It apparently didn’t take too much speed to start track designers thinking about the superelevation process. In Grand Haven, Michigan, the Grand Trunk Western’s passenger station was a stub-ended affair, with one platform about five passenger-car-lengths long. Within 200 feet of the end of the bricks, the track began a long curve–and that track had a definite superelevation to it! How much speed could anyone have gotten up in that distance? (Don’t know about degree of curvature, but the radius was about 1100 feet, give or take 100.)
Well, I don’t know that I’d design the railroad around unexpected emergency applications, if that would involve a huge change. But, they do happen in the darndest worst places - one of the corollaries of “Murphy’s Law”. With the fail-safe, plan-for-the-worst mentality of most railroad designers, yeah, as a design philosophy we probably would. What should we do instead ? Say, “Well, we’ll just shut down the railroad if an unexpected emergency brake application happens to occur there” ? Or, “We’ll accept the chance of the now-known risk of a string-lining derailment occurring here in this curve if that happens” ? Don’t think so.
Fortunately, the choices and results are not that extreme. The start-up pulling forces after an unexpected stop cannot reasonably be expected to exceed the strength of the couplers or the allowable tractive effort of the motive power, and that force is just about the same amount that would be present and have to be planned for in a slow-moving train dragging around the same curve. Said another way, the pulling force when starting as compared with when moving slowly under maximum traction are not hugely different - trains don’t accelerate that fast, so the acceleration component of the pulling force is pretty small in comparison to the overall pulling force needed just to keep the train moving. Recall one of Newton’s Laws of Motion - the result of that law is that the same force applied to an object will keep it in equilibrium - either in a state of rest, which is a train starting with small acceleration - or at constant velocity, which is the same train now moving around the same curve at any constant safe speed. Conclusion: We design the railroad’s curves and superelevation and write the train make-up and handling rules to be able to stop on the worst curve and safely start again, or safely move