Yup. Brake Pipe Flow Meter.

This type was common on road units purchased by SP between about 1960 and the mid-1970s. As an increase in the rate that air was being fed from the locomotive into the brake pipe was detected, a little black pointer moved from left to right across the face of the meter, which had numbers on it. When the pointer reached a certain point, the amber light would illuminate. As the air flow decreased, the pointer would move to the left and the light would go out. The silver knob controlled the red pointer you see, which was used as a mark so the engineer could detect any movement by the black pointer after it settled down.