Pretty sure it goes on the screw that attaches the draw bar to the loco. Put the screw through the draw bar then add the spring. Edit: having just see a picture, what I have said is absolute rubbish. I believe it attaches to the tender end of the draw bar then attaches to the loco. The hole in the loco end of the draw bar is elongated so the spring allows the draw bar to be long when the loco is moving but closes up when stationary.