Car Weight

Car Weight

Weighting Your Rolling Stock

What has probably become one of the most controversial recommendations of the NMRA is RP-2O.1 Car Weight. An RP is a Recommended Practice. It has been created through testing and is a suggestion made to help your rolling stock perform better on both your layout and that of others.

Why is adding weight necessary?
For many years (and still today) there have been great differences in the weight of cars from different manufacturers and those scratchbuilt by individual modelers. Light cars in a string of heavier cars can be pulled off of the track in a curve because they don't have enough weight. Yes, the prototype can have the same problem, but their cars still weigh a lot even when empty. Ours have only the weight we give them. Right out of the box, car weight will vary from manufacturer to manufacturer and even within the same manufacturer.

Through testing, it has been shown that performance can be improved even over poorer trackwork when the car has decent wheels and is weighted to an optimum weight. Since the tendency to derail on curves is greater the longer the car is, the weight will vary with the length of the car.

Can't cars of lighter weight perform just as well?
They may very well perform well on good trackwork and with good wheels that roll well. If you have no older cars and everything on your railroad is very good, you may not need to bring your car weight up to the NMRA recommendation. However, you will still need some formula for car weight to help those cars be consistent performers. I have seen some beautiful new cars with no weight that were only good performers when sitting on a siding and that's no fun.

Well how about more weight? Wouldn't that be better yet?
Greater weight than the recommendation will seldom add to the ability of the car to roll downgrade and track well. What it will do is to make it harder to pull your train up grade. Any additional benefit is offset by wheel/axle friction. Also, and just as important as having the best weight for a car is the matter of keeping it as low as possible. You do not want a top heavy car!!

The below table is reproduced from RP-20.1. To find the optimum weight of a given car, select your scale and find the "Initial Weight". Then take the "Additional Weight" and multiply this by the number of actual inches in the length of the particular car. Add this weight to the "Initial Weight" for the total Optimum Weight of the car. In HO, a 6 inch car should weight 4 ounces. That is a 1 ounce minimum plus 1/2 ounce per inch of car. 1 + 3 = 4 ounces.

per inch of car body length
O 5 +    1
0n3 1-1/2 +    3/4
S 2 +    1/2
Sn3 1 +    1/2
HO 1 +    1/2
HOn3 3/4 +    3/8
TT 3/4 +    3/8
N 1/2 +    .15

Some people will say that the NMRA RP is outmoded and obsolete. Perhaps it may appear to be on a given railroad under specific conditions. However, when interchanging with other modelers, it is the one best way to enjoy running trains without constant derailments of your or the others cars. Many clubs require any car run on their layout meet the NMRA recommended weight. I have found it to be a distinct advantage on my own railroad in allowing me to run both older cars and newer ones together.

What we (the NMRA) have not done is to convert these figures to Metric. Perhaps someday we will.

Well, that is all well and good, but how do I add weight?
There are probably as many ways to add weight as there are modelers and cars needing weight. So much depends upon the car are the space available. In my case, I have a small diet scale that I use to weigh my cars. Some people buy an electronic postal scale to be absolutely accurate. I am happy with close. I have three weights that I know the weight of and I use them to calibrate my scale.

My favorite boxcar weight is a combination of the oval fishing line weights. I mash them with a hammer to get a flat side suitable for gluing and glue them inside the boxcar out of sight near the ends. Be sure that the weight is balanced from the ends and centered in the car. For glue, I use Goo, CA (Super Glue), or and adhesive caulk & seal. There is a low temperature weighting material available for those who prefer that. I have also seen buck shot, bird shot or BBs glued into the center sill of flatcars and inside the bins of covered and open hopper cars. Others use pennies, but I prefer to spend those rather than use them for weight. On one small flatcar, I have a white metal diesel power generator load that is an almost perfect weight. In other words, you can be as inventive as you wish to reach the desired goal.

Roger Hensley

Roger Hensley holds the following certificates: Association Volunteer and Model Railroad Author, as well as the Golden Spike Award.

Henry Reeves shared his online calculator using the formula shown above.

Page last updated December 04, 2018

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