I'm new to this hobby, and have a question that I bet has been asked many times.  I have two HO sets, and one N.  All the trains are Bachmann.  Most of the HO couplers are the same, but I have received a few cars with different couplers.  Is there a quick and easy way to get and change the couplers so that they all match?  How do I tell which couplers I have and which couplers I need?  All the N couplers are the same, so there is nothing to change with them.