There are many things that need to be done to keep your trains running smoothly. Good wheels, couplers, trucks and of course track are all critical. Proper and consistent weight in each car is also important. The longer your trains, the more important proper weight becomes.
How much should a car weigh? The National Model Railroad Association has established a set of standards for car weights. In short, no matter what the scale, longer cars should weigh proportionally more than short cars. Over-weighing is just as detrimental as under-weighing.
There is a common misconception that if adding weight helps with the way a car tracks that adding a lot of weight will overcome additional rough spots in track. Others maintain that if your cars have proper wheels and couplers and your track is all properly laid that no additional weight at all should be required.
In practice, no amount of weight will compensate for a poorly laid track, out of gauge wheels or a low coupler. Also, trains of lightweight cars will track well enough if all other factors are correct, but mixing light and heavy cars can cause more issues.
The NMRA practices are a proven standard that works well in every scale. Whether you choose to follow that advice or go a little heavier or lighter, be consistent across all of your cars.
Many cars already include a steel weight. It still may not be enough to bring the car up to the recommended standards. An inexpensive postal scale makes it easy to keep accurate weights. Whether you use the NMRA practices or not, keep weights consistent across your roster.
Almost anything can be used for weight in a freight car. Lead fishing weights, b-b's, pennies, sand, even kitty litter (unused!) can all be added to closed cars like boxcars and covered hoppers. Avoid metal shavings which could spill out of the car and then be attracted by the magnetism of your locomotives' motors.
Whatever you use, keep the weight as low as possible on the car body. A low center of gravity is very important.
It is also important to secure the weight to keep it from shifting. Use a plastic-compatible glue. White glue often works well, especially on loose loads like gravel.
Adding Weight to Open Cars
Sure, adding weight to a boxcar is easy, but you can't put a stack of pennies in a gondola without turning a few heads. Weighing open cars like flatcars, gondolas, and hoppers can be a challenge.
An easy way around the problem is to add a load. The load itself could be the weight, or it may just be a way to hide the weight inside.
For empty cars, a false floor or concealed weights tucked underside sills and inside frames may be your only options. Simply adding free-rolling metal wheelsets will greatly improve the performance of any car. Substituting metal trucks for plastic can also add some extra ounces and maintain the low center of gravity without modifying the car itself.
When all else fails, painting the weight the same color as the car will help hide it from view. For example, fill the bottom of the bays of a hopper car with lead shot, glue in place and paint it flat black. For added effect, you could sprinkle a little scale-sized coal on top to represent a partially unloaded car.
Too Much Weight
You can overload a freight car. Adding too much weight will limit the number of cars your locomotive can pull. They also put added stress on couplers and their springs, especially on grades. It can also put added stress on the cars trucks and wheels or bow the frame. This, in turn, can offset the height of the couplers creating even more problems.
More commonly, heavy cars create issues in train handling when mixed in a train with lighter cars. If the lighter cars are ahead in the train, the extra drag will cause the light cars to "string line" or pull off the rails towards the center of curves.
And if heavy cars break away on a grade they are apt to do more damage to whatever brings them to a stop than a lighter car.
Most models will be underweight or at proper NMRA weight by default. Older metal models, or if you've added a heavy load, can come in over weight. In the latter case, it may be advisable to remove the factory weights from the car. Otherwise, look to the underside of the car for places where you can file away some of the metal frame or floors where the loss of detail won't be noticed.