Valley of the Sun is a booster name for the Phoenix area. The sun part is right, but it never seemed like it was in a valley to me. In any case, this 28-page brochure is from 1950.
Phoenix is an attractive place, but it wasn’t on the main line of the Santa Fe, which passed to the north through Flagstaff, or the Southern Pacific, which passed to the south through Tucson. Both had branch lines to Phoenix, and in 1923 the two opened a Phoenix Union Station, and a few years after that SP rerouted the Golden State Limited and Sunset Limited through Phoenix over a slightly longer route than the main line. To get to Phoenix on the Santa Fe, however, required a trip from the main line on a train sometimes called the Hassayampa Flyer, which connected with the Chief with a through sleeper to Chicago.