The Potomac was actually a "Northern" as the loco was first designed and used on the Northern Pacific, hence the real name "Northern". However, I understand that folks can call them Southerns, Easterns, Westerns, Potomacs, Greenbrier; or, Poopsies, if they so desire! One of my favorite Steam Locos was the Timken Four Aces (1111) which was a demonstrator locomotive built by Timken to demonstrate the use of roller bearings on the rods and wheels of locomotives. The Four Aces was purchased by the N.P. in 1930 and became N.P. Class A-1, the only loco in this class. Previous Class A locomotives had been designed and purchased by the N.P. starting in December of 1926. The class As had some problems with the driver bearings freezing up and breaking axles