Seattle-Portland pool service began in the 1920s, perhaps 1926. In the early '20s there were about ten trains a day each way, some with very similar timings.
Originally (late 1920s), I believe, five trains per day each way, including an many-stops daytime local that was gone before the mid-1950s. Overnight train lost mail contract ca. 1959 and ended. When I was very young, my family rode it northbound after getting off the Shasta Daylight in Portland.
Probably the longest train in the 1960s was UP's: northbound with connecting passengers from SP Cascade, UP City of Portland and Portland Rose; also sleeping cars off SP (and UP 105?).