For fan driven smoke, I think you'll find that a significant percentage of MTH locomotives and locomotives equipped with my Super-Chuffer will do this. It relates to a technique for controlling the fan that MTH has a patent on and I had to license to use in my Super-Chuffer. The fan is stopped using dynamic braking and not just allowed to coast to a stop. This cuts off the smoke abruptly and seems to frequently cause smoke rungs to form.
It's the same reason many mechanical smoke units (puffers) throw smoke rings, when the piston reverses, it suddenly cuts off the smoke and allows the ring to form.
Although I've seen this with some Lionel locomotives with fan driven smoke, it's less often, at least in my experience. Lionel doesn't dynamically stop the smoke fan, so the smoke tails off after the chuff and doesn't form a ring.
The speed of the locomotive and chuff rate also have a lot to do with it. My Lionel Hogwart's upgrade with the Super-Chuffer will throw streams of perfect smoke rings at a specific speed, however slower or faster, they don't form nearly as often.
