Introducing the #747 O Scale Type E Medium Underset Metal Couplers, a cutting-edge coupling system boasting New Generation AAR Type 'E' 'Prototype' Head Medium Overset Metal Couplers, along with #850 O Scale centering spring. This comprehensive set includes Plastic Gearboxes and lids, and can be mounted using the integrated #817 Plastic Gearbox or optional #818 Metal Gearbox and #819 Extended Metal Gearbox.