4449 and her 27 1941-built sisters were originally had a single headlight bulb on the upper headlight. In the late 1950s Southern Pacific replaced the single upper bulb with the twin sealed beam type .Why and did southern pacific dislike the design.
My guess is that if you have one headlight and it burns out, you have a big problem. If one light burns out when you have 2, you still have a headlight.