Thermal Expansion and its Effects on Train Lengths

I would think changes in slack would make more of a difference than thermal expansion - several inches per car versus for slack as opposed to a fraction of an inch for thermal expansion. 

Consisted train lengths are calculated from the UMLER (Universal Machine Language Equipment Register) records for cars which list their lenght as a part of the cars record in addition to a number of other facts about the car. 

Within the draft gear of each car, even non-cushioned cars, there is an inch or more travel in the coupler shank within the draft gear on each end of the car.  So each car with the slack stretched out of the train will be several inches longer than the listed length of the car.  Cars with cushioned underframes can be a foot or more longer with the slack stretched.

Despite knowing the consisted length of the train, a good dispatcher will inquire of the train for their most recent Defect Detector lenght when planing a meet where the train length and siding length are a close fit.

Ted Marshall

On more than one occasion, I've noticed differences between offical train lengths and those reported by wayside detection equipment, usually varying anywhere from 5 or 10 to over 25 feet. This discrepancy seems to occur mostly during the day, even more so during hot summer months. Coincidence or am I really on onto something?

We all know about sun kinks and the lengths to which they have been discussed here, and frankly, I wouldn't be surprised if this topic has been discussed to length before as well.  If so, perhaps it's worthy of revival. Either way, I suppose inside I seek validation to my theory.

Thanks. 

The thermal expansion coefficient of steel is 0.00000645in/in/degree Fahrenheit.

 For a 10,000 foot train, the difference between -30 degrees in North Dakota in January and +120 degrees in Arizona in August is 150 degrees x 12 x 10,0000 x 0.00000645 =  116" = 9.675'

But "nominal lengths" of cars over pulling faces are not measured in North Dakota in January at -30, they're measured on a ~70 degree F day at the car builder, so the difference is only ~ 50 degrees or 3.225' between the average day and the hottest possible day.  And this is for a 10,000-foot train.  Few trains are of that length.

Detectors are not designed by the manufacturer to be accurate more than +/- 1-3 feet over a 10,000 train length -- and may not be even that accurate in the field.  They do not actually measure length; they measure time and speed between two induction coils and calculate length.  Error occurs as train speed varies while it passes the detector and slack runs in and out.  If a detector comes up with a length 100 feet different than the train sheet, that might be worthy of note, but 10 or 20 feet variance is not significant.

RWM

Once in a while on run through trains coming off a foriegn RR I'll find some of their paperwork.  It's not unusual to see a different length or weight, despite no changes in locomotives or cars.  One railroad had two lengths.  One for the slack stretched out and one for it bunched up.

Jeff

Jeff, have you noticed that your own trains may have gotten shorter?  On paper, anyway.

For most of the time since the merger, UP, in its UMLER screens, rounded the car's length upward to the next full foot.  On a coal train, where cars have a universal overall length of 53'1", for example, everything was shown as 54 feet, so you'd gain 11 inches per car.  On a 132-car train, that would be 110 feet!  However, recently I've been seeing more accurate lengths given.

Jeff, have you noticed that your own trains may have gotten shorter?  On paper, anyway.

For most of the time since the merger, UP, in its UMLER screens, rounded the car's length upward to the next full foot.  On a coal train, where cars have a universal overall length of 53'1", for example, everything was shown as 54 feet, so you'd gain 11 inches per car.  On a 132-car train, that would be 110 feet!  However, recently I've been seeing more accurate lengths given.

Jeff, have you noticed that your own trains may have gotten shorter?  On paper, anyway.

For most of the time since the merger, UP, in its UMLER screens, rounded the car's length upward to the next full foot.  On a coal train, where cars have a universal overall length of 53'1", for example, everything was shown as 54 feet, so you'd gain 11 inches per car.  On a 132-car train, that would be 110 feet!  However, recently I've been seeing more accurate lengths given.