I always feel they look very similar, but I wonder how much of that is just being unfamiliar with them so I don't spot the more subtle differences. For example someone who isn't an enthusiast might think that a 37 and a 40, or a 40 and a 45, look pretty similar to each other- and it's only because I've been interested in knowing the difference for most of my life that means I can tell the difference pretty much automatically. An American enthusiast would be able to identify a particular US loco much quicker than I can, but might not instantly spot the difference between a 40 and a Peak.