This post is so timely for me. I have all scale equipment, but I just got these 15" k line aluminum cars and they are perfect on my layout. My 18" are ok as well, but for the size of my layout the 15" cars look terrific.
Also, for me it's tough to differentiate the difference between the 15" cars and pictures and videos I have seen of the fleet of modernism cars, which look short to me.