I have heard speculation it was a headlight for backing but as Tim said it might have been a lantern.
For your trip as you drive west from Georgetown take the first exit, to the left is the Depot, the enginehouse for the Georgetown loop and the display house with 3 C&S cars. To the right about two blocks the 1006 is on display, this is missed by many Georgetown Loop visitors. The Round window is no longer part of the cabose.