GGG already pointed out the original problem, but to put it in perspective, modern transformers like the z1000 or the TIU it's self don't actually lower the voltage. Instead they just turn the power on and off really fast. the problem arrises when you put two of these devices in a row because they can be out of phase with each other, cutting off different parts of the wave form.
If you use a non-electronic transformer such as a postwar type ZW or KW or whatnot it will work as you originally wanted where you could limit the top speed with the TIU and the transformer will still vary the engine speed.
