Help me understand this operating rule.

[n2mopac]

In the April '05 Model Railroader (pp. 88-90) one column discussed prototype compliance that can easily be converted to model railroading operating rules. One prototype rule discussed was, and I quote, "Long cars (80 feet or more) must be at rear of train if coupled to cars of less than 45 feet." My question is why? I understand that long equipent when hauled empty should be near the end fo the train so heavier equipment behind the car won't pull it off of the track in curves, but how do the 45- length cars figure into this equation. I am interested to get and expert's insight into this. Thanks.

Ron

[zardoz]

It has to do with the amount of overhang the long cars have beyond the trucks. The ends of the long cars swing out much further than that of a short car when going around a curve.

The "pull it off the tracks" you refer to sounds like the "stringlining" problem, where a car or cars is pulled inwards on a curve if subjected to excessive tension between the front and rear of the train. The overhang problem I refer to is the opposite effect: the long cars will lift a short car towards the OUTSIDE of a curve. In addition, the risk is much higher if there are long cars on each end of the short car.

[dehusman]

It also varies by railroad. On the UP you can't have a long (89 ft+) and a short car (39 ft-)coupled together anywhere in the train (other than a boom car for the wrecker and the car coupled to the caboose.

Dave H.

[BaltACD]

Long car - Short car will cause an action known as 'String lining' within a train as it opeates in terrain with much curvature.

The railroad curves and the combination of Long-Short will pull the string straight...thus derailing the train.

[csxengineer98]

its not just an issue of curvers on the main line.... when you have a long/short car next to one another will also cause a derailment just trying to navigate the switches in a yard....i have seen this happen first had on a few occations...
csx engineer

[BaltACD]

QUOTE: Originally posted by csxengineer98

its not just an issue of curvers on the main line.... when you have a long/short car next to one another will also cause a derailment just trying to navigate the switches in a yard....i have seen this happen first had on a few occations...
csx engineer

Negotiating switches constitutes curvature....in yards you are normally dealing to 10mph or less turnouts which constitute a high degree of curvature.

[csxengineer98]

QUOTE: Originally posted by BaltACD

QUOTE: Originally posted by csxengineer98

its not just an issue of curvers on the main line.... when you have a long/short car next to one another will also cause a derailment just trying to navigate the switches in a yard....i have seen this happen first had on a few occations...
csx engineer

Negotiating switches constitutes curvature....in yards you are normally dealing to 10mph or less turnouts which constitute a high degree of curvature.

ok..i guess if you want to split hairs...ill give it to you..lol.... but when i hear the term curves..im thinking of just that..a curver...not a switch....
csx engineer

[csxengineer98]

oh yea..its not so much as an issue of sting lineing...string lineing mostly takes place when cars are pulled off a curver becouse of to much head end pull... basicly taking a curved sting and pulling one part of it will cause it go to strigth.... long/short cars cant be side by side more becouse of latteral froces of the draw heads... the long cars have longer draw heads then do the short cars... and when you go around a curve..becouse of the more swing movement of a longer draw head..it can basicly force the short car its hooked to right off the rails becouse of its shorter drawhead...and wheel base....
csx enigneer

[Anonymous]

QUOTE: Originally posted by csxengineer98

QUOTE: Originally posted by BaltACD

QUOTE: Originally posted by csxengineer98

its not just an issue of curvers on the main line.... when you have a long/short car next to one another will also cause a derailment just trying to navigate the switches in a yard....i have seen this happen first had on a few occations...
csx engineer

Negotiating switches constitutes curvature....in yards you are normally dealing to 10mph or less turnouts which constitute a high degree of curvature.

ok..i guess if you want to split hairs...ill give it to you..lol.... but when i hear the term curves..im thinking of just that..a curver...not a switch....
csx engineer

Now....when I think curves it has nothing to do with tracks or trains...lol....

Daaaaaaaaaaammmmm BABY!!

LC

[csxengineer98]

QUOTE: Originally posted by Limitedclear

QUOTE: Originally posted by csxengineer98

QUOTE: Originally posted by BaltACD

QUOTE: Originally posted by csxengineer98

its not just an issue of curvers on the main line.... when you have a long/short car next to one another will also cause a derailment just trying to navigate the switches in a yard....i have seen this happen first had on a few occations...
csx engineer

Negotiating switches constitutes curvature....in yards you are normally dealing to 10mph or less turnouts which constitute a high degree of curvature.

ok..i guess if you want to split hairs...ill give it to you..lol.... but when i hear the term curves..im thinking of just that..a curver...not a switch....
csx engineer

Now....when I think curves it has nothing to do with tracks or trains...lol....

Daaaaaaaaaaammmmm BABY!!

LC

lol..... them curves i do like... lol
csx enigneer

[kenneo]

The problem that csxengineer relates is not JUST a curve. Nor is it JUST a switch. The problem is a REVERSE Curve without a straight strech equal to or greater than the length of the longest car in the train. Not only do you have the problems related above with drawbar overhang, truck center to coupler draw face distance and going around a curve (the sharper the greater the problem), but you have each car going around two distinct curves in opposite directions at the same time.

If you can't picture this in your mind, you have a switch into a yard throat or a siding, say, to the right, the first car turns right into the diverging route and then within just a few feet, begins a left turn. Real quick, you have the first car going left and right, and the rear car going right and left, as they negotiate the route. This is guarenteed to "pop" the short car off the rails. Yard switches are particularly prone to this because they are sharp curves and slow speed.

As csxengineer, BaltACD and zardoz have explained, also to be factored in is the location within the train in relationship to the road and any helper power. This can force a car to be "popped off" by pushing too hard or "pulled off" by pulling too hard. An accident that you should be aware of where a "pull off" or "stringlining" occurred was the derailment a few years ago where an SP train, full tonnage and grinding uphill slowly between Dunsmuir, CA and Mt. Shasta, CA, pulled an empty "short" car within 5 cars of the road power off the track causing a HAZMAT tank to fall into the Sacramento River with the resultant spill killing everything in the river almost all the way downstream to Lakehead. Very bad thing.

[dldance]

The recent San Marcos derailment on UP had box cars coupled to tank cars. The box cars were much shorter than the 80 footers, but still coming through an S-curve created enough lateral force to derail 7 cars. The physics boys will have fun simulating that one!

dd

[n2mopac]

Thanks for all your help. If I understand what y'all are saying, the problem is when a car over 80 feet is coupled directly to a car of 45 feet or less. Is this correct? If so maybe I misunderstood the rule. The way it was stated in MR was, "Long cars (80 feet or more) must be at rear of train if coupled to cars of less than 45 feet." I though they were saying if 80+ cars and 45- cars were in the same train, but this relates to being coupled directly together I take it. If I am understand correctly, then wouldn't it be more logical to say that 80+ cars and 45- cars should not be coupled directly to one another? Also, why would they necessarily need to be at the end of the train, and why would being there lessen the phenomena y'all describe above?

Ron

[kenneo]

Long Car coupled to Short Car --- should not be done if avoidable.

If not avoidable, then placed at rear of train and behind any helper power. The less "pull" (trailing tonnage) there is, the less likelyhood of the short car being pulled off the track.

Also, if the short car is a load (say 100 ton cement hopper - 36 foot car over coupler faces), it will tend to roll the rail over on its side rather than be pulled off the track. Remember, the critical dimension is the distance between the two cars truck centers as measured from the center pin to the nearest coupler pulling face. This distance, rather the difference in the distance on each individual car, is what causes the trouble.

As mentioned above in a couple of the messages, the longer the car, the longer the distance between the center pin and the coupler face. That means, that on cars such as 89' pig flats, the coupler can actually be outside of the railif the curve is sharp enough where the coupler of the cement hopper will still be over the center of the roadbed. This angle difference causes the short car to derail by lifting it up and over the high (outside) side of the curve. When the short car derails, it also derails the long car next to it and then both cars quickly move to the low side of the curve in a "stringline".

Now, picture that going through a reverse curve as described in my note above, and you have cars going everywhich direction at the same time. Should the train change its TTD profile at that moment (accelerate or decelerate) all kinds of havoc occurr.

You can demonstrate this on your moddle railroad if you have at least one very long car and one very short car with FRAME mounted coupler pockets. Also, take a very long cut of cars (your 40 foot-60 foot models will do) and pull them through a reverse curve from the main into a siding and the cars at the head of your string will derail due to the adverse Track-Train Dynamics (TTD). In this case, they will "stringline". It is hard to get the real life cars to "stringline" as neatly as this, but when you combine the two forces, you have the situations the rule is designed to eliminate.