You can control TMCC or legacy locomotives on as many loops as you want with just one Cab-1. Just select an engine (ENG 5 for example) and now you're controlling Engine 5. The others will continue to run at constant speed until you select them again. ENG 3 will switch the Cab-1 so it is now controlling Engine 3. Engine 5 will continue what it was doing until you select it again, etc.