Use of different transponders is a bit vague when you look at the EZ Pass website for different states. Some states indicate that you don't need a separate transponder when towing and that when you go throught the toll booth, it will detect the trailer and charge accordingly. BUT, some of these states state that you should check with other states when traveling. I've used my transponder for just my truck (no additional axles) in other states and haven't had a problem. I can't say for sure whether I was charged for extra axles or not. If I were signing up for the first time, I think I would pick a state that doesn't require a separate transponder and just use it!