Pretty sure the ore car taconite extensions were welded on steel parts, not wood. Wood wouldn't last too long. Taconite pellets can't be heaped up to extend over the top of the ore car sides the way wet, sticky natural ore can, so if you fill a standard ore car to the top with taconite pellets it won't be up to it's maximum weight capacity. Adding the extensions to the existing steel ore cars allowed them to carry the same weight of taconite pellets as they had of raw iron ore.