After making the 7 loads using the plywood they did seem a little heavy and added drag. I was able to find a 2'X2' piece of foam at Lowes for about $6. So I decided to start over. ( after all I have a 50 lb. bag of coal) . What I'm showing here is the difference in the weight using the plywood vs Foam.
This is the plywood base.
This is the foam base, over 1/2 lb. difference in just 7 cars.
I'm having so much fun making these I've filled 13 hoppers and 6 gondolas.