Alan, there have been other 6 axle passenger units which have been successful, such as EL/NJT U34, and the P30CH. Between my example of the FP45, and SDP40F, it still begs a few questions. One would be the wheelbase of the Flexicoil, vs. HT-C truck. Was the hollow bolster an issue? How about the above board water tanks? One theory was that sloshing water made them unstable.
I forgot to mention the SDP35, 40, and 45. I haven't heard of Seaboard, Southern Pacific, Union Pacific, nor GN/BN having any issue with them.