My vote for the most improved looks goes to the K37s. They were ugly standard gauge 2-8-0s but look great as narrow gauge 2-8-2s. In fact they are my favorite engine of all time. I have been watching them since I was a little boy growing up just south of La Jara. They make the narrow gauge look very narrow, are big brutes and can pull the heck out of pipe trains. They are truly unique.
Woody