I think the command units use those. Command gets a constant 14-18v at the track (mostly 18v) Holding the loco so wheels spin & it doesn't take off like a rocket, note what voltage does make smoke. (Is that enough smoke? Lower ohms get hotter per volt)
If its only a few volts difference. It may take a minor wiring change, but you could then insert diodes for a voltage drop to the motor, full power will go to the element. ....All as an alternative to chasing down another element of course Ive done this to 4 locos to improve smoke with the elements on hand that weren't exactly correct, or were poor smokers when new.
Read amps the motor draws, 4a may be enough, 6a definitely large enough and will stay cooler, but theytare bigger. Choose general purpose rectifying diodes of your amp choice. Use in pairs in opposed directions & pair legs tied together, in series with other pairs, to the e-unit input from roller wire. Each pair added eat about 0.7v. So six diodes, 3 pairs in series eats about 1.8v-2.5v +/- a few points. (results vary slightly by diode choice) ten diodes would give a little over 4a drop, etc, etc.
The smoke unit gets connected between diodes and roller so it ends up with element getting a full 11.5-12v out of 11.5-12v, while the motor sees only 9.5v. of 11.5-12v thru the diodes.
GP rectifying diodes are super cheap if you shop (literally pennies ea.) and useful for lots of things in the hobby.