The 79 MPH restriction is actually for trains without some sort of automatic train stop. Most railroads in the US used General Railway Signal's IIATS. The ICC law requiring it on trains traveling over 79 MPH only went into effect in 1951, so there was a surprisingly small amount installed. The Santa Fe, for example, was the only one to equip their Chicago-California lines. ATS was never popular because of the maintenance the system required. Most trains were officially scheduled to travel under 80 MPH, but of course sometimes the rules were bent. Rail passenger service was being pinched by this time.