Abstract: A model train coupler for coupling model train cars. The coupler includes a knuckle pivotally carried by a coupler head, and an internal locking mechanism for locking the orientation of the knuckle when closed. The knuckle defines an arcuate configuration such that engagement between two such knuckles securely couples the two cars on which they are mounted. The locking mechanism includes a locking arm carried by the knuckle and a locking ball. Both the locking arm and the locking ball are received within the coupler head. The distal and of the upper member defines a recess for receiving and limiting the rotation of the locking arm relative to the coupler head. The upper member recess defines a receptor for receiving the locking ball entirely above the locking arm. When the locking ball is encouraged into the upper member receptor, the knuckle is free to pivot. The lower member defines a corresponding lower member recess for partially receiving the locking ball when the knuckle is in a closed orientation.