Although my collection includes at least ten different kinds of Southern Pacific on-board stationery, none of them are specifically marked for the Shasta Daylight. At least in its later years, it is likely that passengers aboard the train would use generic SP stationery.
Here is a piece of stationery from the Shasta Limited, which operated in the same corridor from about 1895 until it was replaced by the Daylight.
Click image to download a PDF of this piece of stationery.
From the 1930s through the 1950s, the SP liked to call itself “the Friendly Southern Pacific.” This stationery lists all of the railroad’s streamlined trains, dating it from after August, 1950, when the overnight Cascade was put into service on the same route as the Shasta Daylight.
Click image to download a PDF of this piece of stationery.
This stationery is even more generic, with a generic train running across the bottom of the letter and the words “Southern Pacific” across the top.
Click top image (Southern Pacific) to download a PDF of an envelope and the bottom image (train) to download a PDF of the letter.