I like the plug cutter idea but don't have any spare sleepers.I ended up buying 16mm beech dowels as they were available and precut to 9cm lengths. Since I need about 50 that's reduced the number of cuts needed by 1/2 as I will just need to flush cut them with a Japanese saw.
Beech will rot away, as it isn't a durable timber.
I would hope that your softwood sleepers have been tantalised. If so, then yes.But then, if you bore into them you'll go past the depth of the tantalising, so stopping water getting into that point is a priority if you want things to last.
Would sleepers react to tantalising anyway?