One thing I have done is coal. Instead of weighing down coal cars with model coal or buying a coal load and making a glue/water mixture and dropping it on the coal to dry it, is, I bought some of these very light green foam blocks from an arts and craft store. It is used to put fake flowers in it and make an arrangement.

I measure out the dimensions of the wagon, cut that soft foam to exacting measurements, paint it black and then sprinkle left over coal onto that paint and let it dry into it. I found this foam on sale for pennies because someone squeezed it too hard and put finger prints in it. It gives a look full load of coal, glistens like coalĀ  and does not weigh down the car.