A thin 4x2 plate with only the outer 4 cyclindric connectors, designed to hold two Duplo train tracks together.

The 2nd example pictures shows how this model connects https://www.printables.com/model/73798-lego-duplo-train-track-ramp-v2/ and https://www.printables.com/model/73784-lego-duplo-train-track-straight-elastic-full-size