I received a comment on one of the earlier posts about the dual MU sockets on the Roy Hill ES44ACi units asking what the story was with them.
Well, the Roy Hill units have dual MU sockets and dual dummy sockets, very much like both Rio Tinto's ES44DCi and ES44ACi locomotives all have the dual MU and dual dummy sockets whilst the Rio Tinto / Hamersley Iron 9-44CW units have dual MU and only a single dummy socket.
I can only think this reduces the need to reach across a coupling to plug in or disconnect the MU.
Roy Hill ES44ACi unit RHA 1012, serial number 62584, built November 2014 at Erie, Pennsylvania USA, shows the dual sockets and dummy sockets during its delivery.
If you know more, please feel free to leave a comment, thank you, Pilbara Railways.