Caption: A locomotive's identity isn't just the unit number - that can change multiple times over a unit's life as it changes ownership. Rather, it's the serial number of the frame (or "frame number") that railroads and roster keepers use to identify lineages of locomotives throughout their lives. GO 604's identity is rather clear, with it stamped right on the official Motive Power Industries builders plate as well as welded to the frame on its own piece of stamped steel: serial/frame number 2405-05 (for 5th unit of the batch of 27 ordered), model MP40PH-3C, built November 2007 at Boise, Idaho.