play with once Is Broken
(I have updated this post. See below.)
To play a track just once and then stop you're to do something like this, using the once modifier with the play command:
Well, that doesn't work in iTunes 12.2. The tracks just keep on coming. In fact, a few times some random track in some other playlist started playing afterwards or queues up in Up Next.
UPDATE (July 1, 2015, 6:39PM) - I think I may be wrong about this. I cannot replicate the original set of issues I describe above.