Take both control arms and the case off.
http://pictures.olsenstoy.com/cd/transfmr/pskw4.pdf

Make sure the contact arm for the throttle in question is touching the coil assembly, and that the carbon roller is not broken.
http://pictures.olsenstoy.com/cd/transfmr/pskw6.pdf

Larry

If you look at the pictorial diagram of the mechanism, you will see both contact arms are insulated from each other by the outer and inner shaft assembly. Both contact arms are wired separately to their respective binding posts.
http://pictures.olsenstoy.com/cd/transfmr/pskw6.pdf

On the contact arm that is not working, double check the wiring from the arm to its respective binding post.
With an AC voltmeter, attach one lead to any 'U' post, and touch the other lead directly to the coil near the contact arm in question. You should get a reading. Move the voltmeter probe across the winding back and forth and you should see the voltage go up one way, and down the other way.
Now take the voltmeter probe and clip it to the problem contact arm near the roller and move the arm back and forth. The voltage should vary up and down like before. Next, move the voltmeter probe to where the wire gets soldered to the arm and check as before. Finally, move the voltmeter lead to the corresponding binding post and check back and forth once again. If you lose voltage anywhere, that is where your problem is.

Double check that all binding posts on the rear terminal plate are solid and not loose. The binding posts are riveted to the terminal plate, and after time and use, the rivets break, and the binding posts make intermittent contact.

Double check all wiring to the schematics. Rely on that only.
http://pictures.olsenstoy.com/cd/transfmr/pskw3.pdf
http://pictures.olsenstoy.com/cd/transfmr/pskw2.pdf

If everything checks out and you still have problems, post clear pictures of the mechanism here so we can see exactly what you have.

Larry

Glad you found and fixed the problem.
Enjoy running your trains again.

Larry