All my locomotives get the Kadee # 26 or #46. (The long center shank couplers.) you will find that most locomotives with all the extras like snow plows and the like need longer couplers than the #5.
Sometimes they are hard to get on. Sometimes the draft gear boxes fit right in and other times you'll have to put in the box and the copper coupler spring, then the coupler one piece at a time, then with a pair of long tweezers, place the top box on to cover the assembly and then the screw to get the entire assembly in place before you tighten the screw. They won't fit if you leave the taps in place...Take a hobby knife and cut off the round tabs that protrude from the sides of the draft gear boxes, (both top and bottom tabs from the top and bottom draft gear box) and then they will fit with a screw holding it all together right down the center of the coupler. Sometimes, i'll also have to trim off some of the back of the draft gear boxes itself to get them to fit.