The last I hear-- after the SDP40F had been retired from Amtrak service-- was that nobody was sure WHAT the problem had been. With the units out of passenger service (and with Santa Fe, the one railroad which bought them second-hand for freight, never having imposed speed restrictions on them (being confident of the quality of their track)), probably interest in investigating the problem died down. If the problem WAS something specific to the SDP40F design, then it probably wouldn't have affected the F40C, whose geometry and weight were more like a standard freight SD40-2.
When the SDP40F was ordered, some influential people probably thought (and, in the anti-passenger-rail government of the day, hoped) that Amtrak would soon fold, so buying a locomotive that was basically a drag freight unit and could be resold for that service made sense. Think about it: start with a six-axle freight unit generally thought ideal for coal trains, now add weight AND length for a steam generator compartment and water tankage, then operate at passenger train speeds on a national network which, in the 1970s, was starved for investment and in many regions scrimping on track maintenance. Whether or not there was something about the SDP40F design that made it particularly susceptible to derailments, the recipe doesn't inspire confidence!