It's the presence or absence of a particular type of signal system which is the determining factor, not the type of operation (schedules, train orders, CTC, etc.). These rules, which predate the FRA by a couple of decades during which time the ICC oversaw things, didn't come into effect until 1949/1950.

The OP asked about SP steam locomotive speeds, to which there are about three answers: what the SP posted in the special instructions for a subdivision (the Cal-P I mentioned previoulsy), a speed limit based on the type of engine, which we didn't get into in this discussion, and how fast a particular engine was able to go. The answer to the subdivision question is based on subdivision and point in time (Cal-P 1914: 30 mph, 1929: 60 mph, 1941: 70 mph). The engine type had to do with driver diameters. Question 3 is pretty much "what you could get away with", and this didn't really change until Gunpow.

After that ICC 1949/1950 order, the maximum speed limit was 79 without cab signals, train stop, etc. This didn't mean you could go 79 everywhere, just that it was the maximum figure the railroad could authorize. Any operation which went faster had to have ATS, cab signals, etc. The Santa Fe had ATS in the Central Valley, but it was removed when the passenger service went away in 1971 when Amtrak took over passenger service (today's San Joaquins started a year or two later). ATS has not been put back (and can't be).