The arrows point to a window apparatus not standard on most diesel locomotives.   Milwaukee Road ‘s #156 was one of the many U. S. railroads theme units celebrating our 200th Anniversary back in 1976.    You can see why it was one of the favorites.

All weather bay windows, like that shows here, were standard on all modern Milwaukee motive power.   They were installed on the engineers side only.    It allowed the engineer to lean to his right and look both forward and rearward,  important when switching,  as it was necessary to see hand signals from crewmen on the ground.   

The window seen here is made of durable fiberglass construction,  and proven to be more user friendly that the earlier plywood and later aluminum versions.  Note that a clear piece of plexi-glass allowed him view the ground immediately below. In darkness, under certain conditions,  it was difficult to judge movement, particularly with moving equipment on adjacent tracks, as in a yard.   A downward shining light enhances this view.  All weather windows allowed the engineer to perform his duties in the relative comfort of a dry cab.

The bay window did not replace the standard cab side windows and common practice was to leave the engineers side windows open, except in extreme cold when they would be closed to minimize drafts.

Radios have eliminated the necessity of an engineer looking for hand signals.  – Gary Ostlund

Mike Schafer and Art Danz contributed to the story