I have two replies, the first possibly more helpful than the second. For the train of scratch built log bogies I have made up logs by laminating cork flooring tiles then shaping with a Surform file (rasp) and a V chisel, dry brushed with browns and brown black to give some bark colouring. They work out to be a moderate weight.
For other set of log bogies, which are LGB ones, I have used suitable prunings from what ever trees/bushes that are about the right diameter and straight enough, which is really is quite a vague unhelpful reply :-[
The logs in the train being hauled by Manawanui are the LGB bogies loaded with prunings. The two sets of log bogies being pulled away from the log skids by the Simplex are some of the scratch built bogies loaded with cork logs. The log skids are made from prunings which I have given a treatment with Metalex, a timber preservative, since they live outside permenantly.