Mechanism that connects the coupler to the frame of the car. In model railroading, the coupler mounting box is often so-named.
                                                            
                                                     
                        
                                                
                        
                        
                            The box under the ends of a prototype car or locomotive (and on most models) where the coupler is spring-mounted to center it and to help absorb shocks and bumps.
                                                            
                                                     
                        
                                                
                        
                        
                            the mounting box or device that fastens a coupler to the car or truck.