The latest issue of SP Trainline has an article on marketing the Coast Daylight. Included is a timetable with listed running times of 9 Hours 45 Minutes in each direction, between Los Angeles and San Francisco. While I do not know the exact rail mileage involved, I recall that the highway mileage via U.S. 101 is 425. Using a little math : 425 miles divided by 9.75 hours is an average of 43.589 Miles per Hour. ( Your mileage may vary, but not enough to really up the average. )
Espee's Coast Line between Los Angeles and San Francisco had precious few places where the Coast Daylight could even run 79 MPH, let alone 90 MPH, and still stay on the rails. Espee's other Daylight trains ( Shasta & San Joaquin ) had even slower average running times due to mountains and curvature.