My guess is that the long wire is the tender electrical connection. There are no visible pickups to the wheels, so I am guessing that the current collection works by the loco chassis being live to one rail with insulated wheels on one side, the tender being live to the opposite rail and a connection between them.
This was a common American form of construction, still used today (my MMI 2-8-2 locos, made in the early 2000s work that way)