Hey Scott,
Some of the newer RTR athearn can also take an 8-pin but all have 9-pin. Genesis should be the same. The Atlas Silver series have an 8-pin NMRA plug.
For an Athearn, it comes with an 8 pin plug, and a 9 pin plug with a dummy board attached to it. You have to take off the dummy board, then plug in a decoder with a 9 pin socket, or plug a deocoder with an 8 pin connector to the 8 pin socket.