The placement of the train number indicator on the top of the smokebox by the stack is an iconoclastic signature of UNION PACIFIC RAILROAD locomotives. It may come as a surprise to learn that this was only one of several locations considered early in the Harriman Era. Until I came upon these drawings, I had never considered the possibility. In hindsight, it makes sense that some experimentation was done prior to selecting the position of the train indicators.
Click to enlarge images.
TOP OF CAB, FRONT
TOP OF CAB, SIDE AND FRONT
FRONT WALL OF CAB
FRONT OF CAB, ON RUNNING BOARD
FRONT OF CAB, NEAR TOP
1945 TRAIN INDICATOR
PIXORAILS 