I think I'm going to go with the Reading Anthracite (ships on Reading Rails, and is a local company). I read somewhere else to use size three (mine run). How many hoppers (approximately) would a bag do?
Or should I be using a different size?
Size 3 is what Brennan recommends for O scale. I'll have to edit my reply above...not sure why I said size 8. If you make them using the rigid foam, you are just putting a coating on top of the paint so figure about ½ oz. per load depending on the hopper size. An MTH die cast will take a bit more and a Lionel 6456 / 6476 a bit less. You should be able to make about 2 dozen loads per bag of Brennan’s Reading Anthracite.