I'm not sure about this theory, but I think adding weight to the cars would also help. I found after adding common sticky tire weights to my rolling stock it quieted them down significantly! Instead of bouncing along they stay solidly down on the tracks. You should try it with one car. Add a few weights, roll it down the track and then a non weighted car and see if you think there is a difference. I certainly do.
Tim Lewis was also highly correct in the addition of cotton or something to stop the reverberation inside closed cars (especially boxcars).
