I have several cars of unknown make which are terribly light. I was thinking of opening them and gluing washers in them for extra weight so they tracked better. Does anyone else do this? Any thoughts on this?
For N scale, you'll want an INITIAL WEIGHT of 1/2 ounce and you should add .15 ounces ADDITIONAL WEIGHT per inch of car body length.
I have Sprectrum cars that weighty more than that new from the box, an example is a single dome car and the triple dome cars. I have several 50' box cars that are much lighter than the Spectrum cars which are maybe 1/4 the weight of the Spectrum cars. They are so sensitive to any difference in track uniformity that it makes them worthless to even try using.
ABC gave you the NMRA-recommended minimum weight for cars and it's an excellent rule of thumb to go by. Until you can add some extra weight, try putting lighter cars toward the rear of the train. That will make them somewhat less susceptible to variations in the track, especially in curves.