@cnwdon get yourself a small bubble level that's only 5"-6" long and move it along the track in the problem area. It will tell you if the track itself is bowed. That's something the loco will react to, which is independent of a level platform, level basement floor, etc.
if you incorporate double-pole, double-throw switches to bypass the rheostats (or to selectively incorporate them for a specific direction of travel), you can easily take them out of the circuit when you're running a low current draw, speed-controlled loco. When the rheostats are in the circuit, you can judiciously pump more than 19V into the rails, because taking into account the added resistance AND the voltage sag you observed due to load, the loco and the electronics inside of it won't be seeing more than 19V.
And as illinoiscentral said above, the way to incorporate a Variac is to put it BEFORE the industrial transformers, varying the voltage to their primaries. That's how the Right Of Way transformer is built. It's NEVER safe to connect the output of a variac directly to the rails!
