That sounds like the first Amtrak train I rode on. SLO-SF in 1973. Something diesel and gray in front (I wasn't a real railfan then), probably something head-end, and a coach, automat, and coach-lounge-obs. Most likely a recycled SP Daylight of the immediately pre-Amtrak era. The next time I took it, Amtrak had switched to Oakland for the Bay Area stop and the train had gotten longer and taken on a rainbow hue (seem to recall a blue - GN? - coach with wheel-powered a/c so it got a bit warm during a station stop in Salinas and at some sidings further south). FWIW it was 5+ hours from SLO to SF or Oakland then, just like it is now.

Wasn't the Westside line on TRAINS' speed survey for a while in the early 1960s? Seem to recall seeing some 70+ mph segments with Dos Palos mentioned, but it would take a while to dive through the boxes and find anything (if the magazines have survived). Would that have been the Owl or possibly a through section of the SJ Daylight?