1. Part of my load testing included hooking up a 15 car train to the engine. That would put both motors under load, correct? Answer: NO, not if one of the motors was slipping and not coupled to the load- most specifically the front motor with the tachometer.
If so, doing this would usually make the engine run well for a few seconds before it just (metaphorically) snapped out of it and didn't move again unless given a push or taken to speed step 50. Again, what is unique is that as you raise the speed steps up (higher throttle) even an unloaded or partially loaded front motor (likely a slipping coupling) then only at some point where a lot of power is needed to spin the motor at a higher commanded RPM does this equal the power of the loaded rear motor to begin moving.
2. Could I be able to tell visually if it is/was slipping? I've taken both trucks off and the couplings look in good condition, and nothing feels loose or out of place on the motor. Of course that could just be me, maybe I'm not looking at it correctly.
Attempt to hold the small coupler end of the motor while gently turning the flywheel. If the coupler shifts on the shaft and the flywheel spins in comparison there is your answer.
It takes till about speed step 30 to start moving, and even then it is still sluggish. Yes, that's exactly how a system with a slipping front motor would operate.
On the rear flywheel motor, I can completely make it stop while moving with just one finger. That should not happen.
NO, you can only say that won't happen if you KNOW and can measure or have some idea of the actual voltage and power the motor is receiving. Again, what you are implying is an RPM and Torque curve based on power input- and this motor is operating abnormally outside of the expected curve.
On example would be put a lightbulb in parallel with the motor. Brighter = more power, dimmer less power.
Again, since this is a "System" we have to understand how the system reacts vs a single element.
If your front motor with the tachometer is loaded, when you command a speed step, the controller does everything it can to maintain that exact speed by raising or lowering the power (I'm not using voltage because it's typically a PWM power rather than pure voltage change) to both motors. Again, if you attempt to slow the front motor, the controller responds with higher power to maintain the RPM.
As a sign of this, if you have the engine on rollers, and load ONLY the front motor (either finger against the flywheel or finger against the wheels) then as you load it harder, the controller will raise the power to both motors, and typically the rear motor will see a speed increase proportional to the load you are putting on the front motor.
If you load the rear motor in the same scenario only, the front motor would just continue to maintain it's current speed/RPM. The controller cannot sense the change in load or resulting RPM of the rear motor, there is no tachometer.
On the track, let's take your scenario of pulling a car load. If the rear motor was weak, then the front motor being the one trying to maintain absolute RPM no matter what would be in wheel slip trying to drag the back end motor and truck not contributing.
If the front motor is slipping at the coupling, the tachometer is seeing the motor spinning at the commanded RPM but the truck wheels would not be slipping. At the same time, because the motor is not loaded well because of the shaft coupling slipping- the rear motor ALSO is not getting much power because the controller is again only trying to maintain the RPM of the front motor.
Again, your symptoms line up to the front motor being the problem.
