I can't say for sure when the last horse car was used for Santa Anita racetrack, but it was around 1970. The standard train to ship horses west on was the Grand Canyon. For Santa Anita, the horse cars were set out at San Bernardino, and, as soon as the Grand Canyon departed, a boiler-equipped GP7 would couple onto the horse cars, make an air brake test, and head for Santa Anita, which had its own spur track and a dock for direct unloading. Occasionally this was done from Los Angeles, but usually not. However, there was Hollywood Park race track in Inglewood, and the horse cars would run through to Los Angeles and be forwarded from LAUPT, down the Harbor District to Inglewood. I think they had to trailer the horses a short distance there.
Also . . . Pennsylvania Railroad also owned some horse cars and sometimes one of them would be billed to Santa Anita or Hollywood Park.