This same question came up in 2008 on the Trainorders site, which I answered, which I am reposting below.

There is an old story that has floated around in the WP community for awhile that the reason WP ordered their early diesel power with Pyle "Ash Can" or "Garbage Can" headlamps was because the railroad had a huge inventory of incandescent bulbs for the F-unit fleet. Unfortunately the real reason isn't quite as enchanting or colorful.

The real reason WP stuck with these Pyle-National C-195-CZ lamps in the early diesel days, as opposed to sealed beam lamps, was simply because because the Pyle lamps were superior. WP was very concerned about lighting due to the variety of terrain they operated in. From the urban regions of the bay area, to the low speed twists and turns of the Feather River Canyon (where rock slides were all too common), to the high-speed running across the Nevada and Utah deserts, WP was very serious about using the best available lighting for safety reasons, especially in the canyon where hitting a slide was deadly and costly to the railroad.

Early (1950s) sealed beam headlamps were not very good performers and WP management was concerned with black spots that developed over time in the sealed beam light beams. The only "locomotives" that had sealed beam headlamps in the 1950s on the WP were the RDCs and they were not well liked.

Also, up until about 1955, all WP trains that traveled between Oroville and Portola (basically through the Feather River Canyon) had their headlamp lenses changed out to what were called "elephant lenses", which were basically fluted lenses that spread some of the light out to the side more, with a slight sacrifice in the distance of the beam. Standardization of lenses was probably desirable so these could be easily changed out as trains passed through Oroville and Portola. For the canyon, a wider beam was preferred as it allowed crewman to spot rock slides and debris on the curves in the canyon better than clear lenses did. At least that was the prevailing wisdom at the time. In the early 1950s, WP tested the effectiveness of the "elephant lenses" and decided the difference between them and clear lenses was negligible so the practice of swapping lenses out in Oroville and Portola was discontinued some time around 1955 and all the lenses from then on were the clear, or "spot" lenses.

All of WPs road power, the GP7, GP9, GP20, GP35 and GP40 models up to GP40 3516 were delivered with the Pyle-National headlamps on the front and rear. Eventually the beam quality and reliability of the dual sealed beam lamps improved and so WP specified sealed beam headlamps on all GE units and GP40/GP40-2 units #3517-3559 and many of the early units were updated with sealed beams or sealed beam conversions over the years.

Most of this information came from Virgil Staff's book "D-Day on the Western Pacific".