There are two (2) main types of couplers that n-gauge train manufacturers install on their train cars and engines. the oldeer type found on the original Aurora Poatage Stamp trains were made by Arnold Rapido and are commonly called rapido type couplers. The other type of copuplers were orginally made by Kadee but are now made by Micro Trains. These are smaller and more realistic and can be uncoupled by backing two cars or engines that have these couplers installed over a permanent magnet placed in the track. Many more serious modelers prefer these magnetic couplers for these two reasons. Roundhouse used to (before they were bought out by Athern) and Kato make couplers that are compatible in size with the Micro trains couplers but do not necessarily work the same way using magnets. When selling n gauge equipment, pictures do not necessarily show enough detail for buyer to tell which coupler is on the item you are selling. So describing the coupler type will help buyers know what they are getting and help you to have an accurate description of an n train product that will more likely not be returned.