Coal Hopper Capacity Questions

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Friday (5/8/15) I snapped photos of four different coal hoppers on a Roberts Bank bound coal train passing the Edmonds marsh. Photos can be seen by scrolling down page 60:

I have some questions regarding the capacity of these hoppers. Note the markings on each car in the photos: Ld. Lmt and Lt. Wt.

I assume Ld. Lmt = load limit = the maximum permissible combined weight of the hopper + the load.

I assume Lt. Wt. = light weight = the weight of the empty hopper, or what we used to call tare weight.

If my assumptions are correct (and please correct me if they are not), then the maximum permissible weight of the coal in each hopper = load limit - light weight.

First of all, I was surprised by the variations in the light (tare) weights of the hoppers. I can understand that the manufacturing of something as large and heavy as a coal hopper may create differences in the weights of individual, "identical" hoppers. I would expect the weight differences to be a small %-tage of the hoppers' weights.

Even though individual hoppers within a class of cars of the same design may vary in weight, I would expect the maximum permissible weight of coal to be identical for each class of hoppers using the formula I typed above. Subtracting load limit - light weight for each of the four hoppers in my photos yields different maximum permissible loads, as shown below:

Former BN Cascade Green hoppers:
BN 533813: 241,800 - 44,200 = 197,600

BN 536262: 242,900 - 43,100 = 199,800

BNSF aluminum hoppers:
BNSF 669290: 244,300 - 41,700 = 202,600

BNSF 671585: 244,200 - 41,800 = 202,400

Note the lighter tare weight of the newer, aluminum hoppers.

How closely does the railroad adhere to the hopper load limits? I would think that to weigh each hopper of several 100+ car coal trains as they are being loaded would be too slow and cumbersome.
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I had the vague impression that weighing cars 'on the fly' may be possible, certainly that each car could be weighed as loaded/unloaded; how else could the customer could be accurately billed?

I'm guessing also that individual lots/manufacturing runs of cars have minor changes in design that affect weight, with the empty weight maybe gradually trending down as those changes accumulate. Possibly each car is weighed as it comes off the line (and is checked every few years) and I'd bet that data is logged in the RFID transponder or a database to aid in the billing process. Also, the weight will change with exposure to rain/snow and even the lost coal dust might amount to a (barely) noticeable loss.

Remember those articulated aluminum coal cars from about fifteen years ago? Wonder how those were weighed? Estimated? Oh well, they didn't work out so well and were eventually scrapped, I think. Just another way to move as much product per train as possible.


New Member

This is a pretty neat video showing them load coal. They don't exactly answer your question, but in the video when they pan over to the right you can see the computer screen and that screen shows every car and what it's load limit is. Each car has a RFID tag which is really just a number for the car that connects to a database that has all the stats on the car including its empty weight and it's capacity/full weight. I suspect that the computer stops the fill of coal based on what the max load weight is of each car. They kind of speak to that in the video.

Also on a side note, in the railroad a car is either empty of full. So if a flat car has a small piece of equipment on it, it's still listed at its full weight on the train paperwork.
Thanks for the video. That pretty much explains how coal can be loaded so precisely. I guess my remaining questions are how the load limit of each hopper is calculated and why the limit varies among hoppers of the same design and manufacture.
The trucks have a specified maximum load. The gross rail load for a pair of "110 ton" trucks is specified by the manufacturer at 286,000 lbs.

Weight the newly built car. That is the LT WT.

Subtract LT WT from 286,000 and that is the LD LMT.

Add the two numbers on the cars you photographed and they all total 286,000.

Variations in the LT WT are due to variations in the wall thicknesses of castings and forgings and in the thickness of sheet steel/aluminum. Given amount of sheet steel/aluminum in a railcar, a variation of a thousandth or two in thickness would change the weight of the car by a lot.

Manufacturing has tolerances established for everything. The tolerances are only as tight as they need to be for the function of the product, within economic reason. At a certain point the cost of manufacturing to tighter tolerance exceeds the value realized from the tighter tolerance, and that's where you establish your tolerance range limits.

I understand cars are re-weighed after major repair work, with the LT WT and LD LMT adjusted accordingly.

Various types of cars are built to different specs. An autorack may be built to a 180,000 gross rail load, with lighter duty trucks to match.

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