My SD80MAC can handle it. It looks ok, but it the pilots do hang out a bit. It doesn't effect operation though. most 6 axle diesels can handle it. I can't really think of any locomotive that can't. its the 80 foot railcars you need to watchout for.
I think most Steam locomotives can handle it, except for those with a long solid frame (2-10-4s, etc). In real life such locomotives would straighten out the rail if the radius was to tight. luckily for you, the models will only jump the tracks. However, i do not see any reason why a 2-10-4 couldn't handle it. usually the manufacturers will tell you what their model can handle. Articulateds ( Challengers, Big Boys, Alleghenies) should have no problem even on 18 radius, but they will look bizarre.