Here's the problem with speed controls:
To vary the motor speed, you need to adjust the voltage up or down.
As you reduce voltage across the windings you reduce current flow potential. Thus you lose torque.
The trick is to keep the ability to flow current to maintain torque (which is what you want/need) while still adjusting the voltage to regulate speed (what you hope to do).
PMDC drives do this by taking the pure DC and chopping it into short duty, full voltage pulses. These pulses result in a lower AVERAGE voltage but, since current flow while voltage is on remains full on/high, torque is not diminished appreciably if at all while current flows.
Lionel trains at one time (my youth) were all powered on low voltage AC. Their train transformers were actually variacs which used a wiper to sweep across the trans former windings to vary the AC from low to high.
Probably a VFD would be what you're in need of. Perhaps someone with more variable AC experience than a Lionel train can help you more here than I.
I suspect something more sophisticated than a rheostat is what you're ultimately going to need....