Many of us have older freight cars with plastic wheels. A major consensus in the hobby is that metal wheels roll better, wear better, and attract less dust and gunk than plastic wheels.
Another advantage is that metal wheels lower the center of gravity and add weight.
Consider a typical HO scale blue box Athearn forty-foot boxcar or equivalent Model Die Casting, Train Miniature or Accurail car. Using the NMRA's Recommended Practice for car weight (RP-20.1) such a car's ideal weight is calculated at one ounce plus one-half ounce per inch of length. That works out to about 3.75 ounces for a forty-foot car.
The stock Athearn plastic wheels with metal axles weigh .15 ounces. This is only 4 percent of the recommended overall weight for the car.
As you can see by the attached slide, various brands of metal wheels weigh two to three times this amount.
As a percent of the recommended overall weight for the car the various wheel brands "weight in" as follows:
So, would you rather have 4 percent of the car's recommended weight down at the wheels or 8 to 12 percent?