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NMRA Standards for Car Weight

The National Model Railroad Association (NMRA) was formed in 1935 to set standards for all gauges in categories including size, weight, power supplies and electrical connections among other things. The larger the scale, the greater the recommended weight. The optimal weight is different for each scale, but is calculated by adding a base, initial weight plus a second figure based on the length of the car. Standardized rolling stock weights are useful for modelers to maintain smooth operation.

O and On3 gauge
- The optimal weight for O gauge rolling stock is calculated by adding 5 ounces plus 1 ounce per inch of the rail car. A 10-inch car should therefore weight 15 ounces. On3 is calculated by beginning with1.5 ounces and adding .75 ounces per inch of car length.
S and Sn3 gauge
- For both S and Sn3 gauge rolling stock, begin with a base figure of 2 ounces and add .5 ounces per inch of car length.
HO and HOn3
- HO gauge rolling stock optimal weight can be found by beginning with a base weight of 1 ounce and adding 1/2 ounce per inch of car length. For HOn3, begin with .75 ounces and add .375 ounces per inch of car length.
TT gauge
- TT gauge is calculated exactly the same way HOn3 is figured: start with a base weight of .75 ounces and add .375 ounces per inch of car length.
N gauge
- To determine optimal weight for N guage cars, start with an initial weight of 1/2 ounce plus .15 ounces per inch.

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