Before and after, 20ft between units didn't amuse me. Red Caboose won't answer email and the unimate couplers I need are nowhere to be found. So I jerry rigged some new McHenry couplers on my IM F units. Right behind the coupler on the shank is a round mold mark, thats where I made my hole. I also added a small piece of styrene to the top of the shank that was even with the top of the coupler so it won't hit the body. THIS IS A VERY FRAGILE PROCESS and I wound up having to glue a few of mine, but they work and that's what matters to me.