Until a couple of years ago, my mainline had 18in curves that were handled quite well by both big engines and passenger cars. It surprised me since some of my engines (Atlas U34CH and Athearn C44-9W) and most if not all of my passenger cars were recommended for bigger curves. The upgrade to 22in curves did pay off when engines found their pulling power increased. My HHP-8 hauled a 6-car train around my layout with ease back in August when it used to strain with a 4-car train.
http://www.youtube.com/watch?v=mDGSGdRp5Zo - HHP-8 hauling 6 cars.
http://www.youtube.com/watch?v=mDGSGdRp5Zo - HHP-8 hauling 6 cars.