You are looking at a vintage Marx lock on. This is the style that sticks into the bottom of the track for a snug fit. It is really the best way to connect to 027 track since they slide in and stay rather than clip. Also used with some accessories.
This one is clean and ready to use. Shows light signs of use and a little bit of oxidation.
Many folks use these for other brands of toy trains as well such as Lionel, K-Line, even American Flyer.