Dale says:

Three things to look at right off the top of my head. First, if it always the same cars going wrong, check that the wheels are actually in gauge and that the trucks rotate freely. Second, while you’re looking at the wheels, look at the flanges to see if they are too big or too sharp (old style pizza cutter flanges) and replace the wheel sets if necessary. Third, check the NMRA standards pages and make sure the problem cars are the right weight. And fourth, look carefully at the points to see if they are being held securely against the stock rail.