I posted this about 3 years ago on Trainorders.com:
"The HTB was a great advance in weight shift compared to the GP swing hanger truck - the weight shift within the truck was nearly zero under adhesion whereas the GP is pretty bad. However, the advent of super series wheel slip control pretty much eliminated the need for a zero weight shift truck. That, combined with the increased cost of the truck and the underframe mods it required, caused its early demise. I am confident the ride would have been fixed if there were enough of them out there. In principle, it has softer primary springs then the GP and more deflection in the secondary as well compared to the GP with compression rubber springs, the standard at the time. I suspect the lateral stiffness of the secondary chevrons was the source of the rough lateral ride, but with some tweaking of the chevron angle, that could have been reduced. But the GP with its swing hangers is about as soft laterally as practical, and the +/- 2.25 inches travel to the secondary suspension stops is huge by comparison to other trucks. I'm not sure what profile wheels the GP40X's were delivered with but when used to pull passenger trains it was recommended they change the profile to 1:40, as used on the F40PH, to limit hunting.
Incidentally, in 1984 the only remaining spare HTB frame in LaGrange was used to fabricate the first prototype of a radial truck, which eventually led to the HTCR truck."
The HTB had rubber secondary springs placed on an angle that focused their reaction at rail level, not much different in principle than the ZWT truck. It did require a different underframe as it used a larger centerbearing than the GP swing hanger truck which was mounted at a lower height relative to the rail, about 28" or so above the rail. And the bottom plate had to be higher because of the increased travel primary springs which were the same as the HTC used. Head to head adhesion testing on the GP40X lococ, half of which were built with GP trucks showed no advantage to locomotive adhesion although there was less difference in tractiion motor currents. EMD was also doing a good business at that time with locos built with trade-in trucks so those things conspired to kill the truck.
Dave