Hi Allen Lee
This rail section was available a brief period of time 1935 only (until Märklin OO adatpted transformers were available)
Here is what I found on the subject
Link https://tischbahn.de/ger...s-anschlussgleis-3600ar/
Here is an interesting pic from that site

We know there were two fabric-insulated wire from the rail.
We see that the variable resistance was connected to the rail frame (see the non insulated lever axle)

The resistor is attached to the metal ballast with a mica insulator in-between.
So logicalling one of the wires was connected to the center rail (still there on the picture while the other is connected to the left of the coiled resistor.
What was the resistor maximum value? I don't know but probably in the values 10 Ω to 50 Ω
The "handle" is a cylinder of red plastic attached to the lever by a long (metric) screw

Just my ideas (I have some imagination and was an Engineer in Electronics and in industrail eletricity). So I may very well be wrong.
Cheers
Jean