I am NOT saying there was never a "unique" truck, that was before my time. But, EMD has 2 test units out at the TTC in Pueblo that were former Amtrak SDP's. They have the "original" trucks and I will be darned if I could tell any difference in terms of appearance, spring types/locations, pedestal jaw measurements, etc. They have the same "HTC" casting emblem like all others that were cast by Rockwell at that time. If there was something "special" about these trucks, from a maintenance standpoint I dont know what it is. Someone willing to examine an EMD 190 parts catalog could research it further to verify.
I am not around the units anymore but suggest you get a close up look for the casting/part number on the truck sideframe on an SDP and then look at a "regular" HTC of the same vintage and compare. Dont jump to conclusions too fast though as I am sure there are many different casting numbers from the 1960's to "today" and with the way trucks get swapped around, even then it may be inconclusive. I do however think it is very safe to say that MANY HTC trucks from SDP's have wound back up in service being purchased from independnt shops as "used/rebuilt".
As info, the 2 former SDP's out at Pueblo run just fine with no speed restrictions and if anything, are even worse for balanced weight at the steam generators has been removed and diesel generator sets and equipment "labs" installed. All this stuff is "off center" on the conductors side to allow a walkway on the engineer side.
Yes, SDP's were "freight engines". From a track geometry standpoint, they should be run at "freight" speed. Most of the time, people think the higher speed for passenger trains is only to reduce travel time. While true and the fact that passenger cars are much "smoother" on crap track, the SDP was not designed as a "true" passenger locomotive. (ala A-1-A or B-B trucks).
6 powered axles are designed to handle more amperage at low speed (below 12 mph) without overheating traction motors. This is never the case on passenger trains due to speed and light tonnage. The extra maintenance of a 6 powered axles (such as maintaining wheel measurements to FRA requiremnts, brushes, gearcases, pedestal liners, brake rigging, et all) makes no sense IF the unit will NEVER go into freight service at any time before getting cut up.
In short, a 6 axle locomotive is basically a waste for passenger operation BUT has tremendous value when it comes time look at resale value. Think of it as buying a 15 year old pickup to pull a large trailer. All else being the same, would you want the one with or without the "trailering package" and would you be willing to pay as much for the the one without it?
I think if you ran ANY SD type unit at passenger speeds back then in the eastern US, it would have suffered the same fate....but that is just my opinion.