It has been many years ago, but my dad had an upright trike. It was one of those "shopping bikes" with a basket on the back. He was still riding regular bikes regularly and used the trike for hauling stuff short distances (shopping and so on).
I don't know much about training wheels on adult bikes, but don't like the sound of the idea. I'd think a fall would be likely on any uneven surfaces if the rider is unsteady enough to really need them.