[Nonfractal]

I'm not surprised that users of magnets struggle with this issue.

Different track types create different down force depeding on the material used to manufacture the rails.
Consequently , cars from one manufacturer don't run well on track from another manufacturer because the rails and onboards magnets are not matched. 

Car manufacturers that don't make track seem to factory-fit magnets to suit one track type.
For example :
Fly cars seem to be set up for carrera stainless track
Slot.IT and NSR seem to be factory set for scalextric and ninco. 
If magnet cars were of interest to club racers (generally, they are not) there would be factory methods to fine tune the location of the magnet inside the car.
The fine tuning would include set screws to precisely lift the magnet away from the rail so as to adjust the downfotce to the optimum.

I don't use magnets in cars because it destroys the handling,  allowing  unrealistic cornering speeds. 
Most clubs forbid magnetraction on safety grounds.

There is more than enough traction to be had from :
a- tyre selection and preparation 
b- magnetic flux leakage from the motor which can be optimised uusing the post-factory offset motor mounts available from some manufacturers.

Front motor cars can be made work well Sans-Mag. 
"Alexis in Greece" uses front motor Fly  cars in proxy races to great effect without magnets.
I'll see if I can find a link and post it here.

Alan W