The SP was always heavier tonnage east than west with containers, lumber, and in the past perishables. Perhaps the UP was the opposite and post-merger they achieved balance. But I cannot imagine that the UP had more tonnage west than eastbound. My guess is that back then the railroad on the west coast was a 5-6 day railroad (as was the SP) since there was little originating traffic on the west coast on Sundays and about half the normal volume on Saturdays, whereas the Chicago gateway would be fairly steady as the traffic there originated on many different railroads and the weekly originating pattern would be offset by interchange traffic. Conrail had a similar problem and would gather up locomotives and empty intermodal cars and run them back west on Sundays to keep Chicago supplied.

My guess is with the growth in international intermodal volume in the L.A. basin that there are now many more Sunday trains departing from weekend vessel arrivals and that has balanced the train pattern much more than in days past.