From the General Code of Operating Rules (applies to all Class A Railroads):
http://www.sdrm.org/faqs/rulebook/signals.html#5.0
5.16 Observe and Call Signals
Crew members in the engine control compartment must be alert for signals. As soon as signals become visible or audible, crew members must communicate clearly to each other the name or aspect of signals affecting their train. They must continue to observe signals and announce any change of aspect until the train passes the signal.
If the signal is not complied with promptly, other crew members must remind the engineer and/or conductor of the rule requirement. If the crew members receive no response or if the engineer is unable to respond, they must immediately take action to ensure safety, using the emergency brake valve to stop the train, if necessary.
I don’t dispute your assertion. What I do recognize is that there are times when these cars are filled to capacity and beyond, standing room only. With all the noise and ticket distractions, I don’t see how the Conductors could possibly comply with this requirement.