Remember too, that after a bad passenger rail accident, the I.C.C. required Automatic Train Stop and / or Automatic Train Control for rail lines operated in excess of 79 M.P.H. After this requirement went into effect, many railroads simply lowered the speed limits on all of their lines to 79 M.P.H. or less.
In the Amtrak era, Santa Fe was one of the few railroads that still ran passenger trains above 79 M.P.H., ( Maybe SCL and IC too ??? ) therefore part of Amtrak's 1st order of SDP-40F locomotives were equipped with ATS shoes, as were some F40PH engines as well.