Uses Bachmann horn hook couplers. Use either the built in pin (unreliable) or a track nail in a 1 mm hole glued in, works better. The three files are slightly different, one is a bit higher so you can wiggle in the coupler without breaking the pin. If you decide to forgo the pin, then use the lower one so you slide in the coupler, then use the track nail to secure it in the desired position. Use an Xacto knife first to trim the inside of the box, since you should print w/ no supports, then trim a bit the open flange. My first design since I needed some of these. There are many experts here, I am sure that someone could do a MUCH better work than I could. I would use the excuse that I am an electrical engineer, not a mech E, but so is OSTycoon an EE and look at the wonderful things he created !