Poplar for subrail construction is best for stability and soft enough to drive staples into. I think Diamond is layering hardwoods like oak and ash for rail construction, but still using poplar for the subrail portion where staples will be used. It's the best of both worlds, poplar is a great subrail material, but where a tougher wood is needed for anchoring the rail to the slate they use a much better grade of hardwood to prevent stripping out of the inserts, or screws like the old Brunswicks had a problem with. I'll be be doing the same with this old Anniversary in my avatar. I'm just going to replicate the rails using laminated hickory and poplar with a cap of walnut.