Looking to quiet noise from cars.

Mostly the third rail pickup rollers is the culprit, you can oil them. The fast angle metal wheels are also noisy. As a test I tried a quality two rail car that had metal wheels and that was a good 50% quieter. I am not sure but I think its because like real trains the wheels  are really polished smooth. The rail on real trains is also polished smooth. You can make tight fitting foam coal loads  for the hoppers, that should help.Boxcars you can put pillow stuffing in them or old socks.Something soft and heavy absorbs sound.   

What type of track were you running on?  Sopme types are more prone to pickup and amplify the noise from wheels ans rollers.

what is the track sitting on? need cork or homosote ,not plywood.hollow rail track is noisey too.-jim

If the hopper makes noise or any box cars you have you can reduce the sounds by adding cotton and taping the doors from the inside. Passenger car might need to be lubed, or all the wheels need to be cleaned.

Thanks for the ideas. Track is Ross and Gargraves on cork roadbed.

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).