He's gone and gotten a piece of a "cut in brush", not the foam type, but the micro fibre sort. He has then glued this to a block of wood that is wider than the ballast area of the HO track. At first I thought it was a clothes brush, but it's better pure genius I say.
So after he has poured the ballast into place and roughly put in in place with a small paint brush, he just uses his new speacial brush to run up and down the track and it cleans all the excess ballast off the ties leaving them nicely finished. All he has to do then is spray some wet water over it all and dribble in the theinned down white glue. Sure is a lot faster than using a small paint brush to pick at each individual piece of ballast.