K-Line F-Series B-Unit Loco Problems

As an FYI, I made sure to clean all of the wheels as well as the power contact rollers with q-tips/paper towels soaked in IPA.  I have also recently cleaned the track with a Scotchbrite pad followed with a paper towel soaked in IPA, so in theory those should not be contributing to the problem.

Now I'm wondering if the fact that it hasn't run for so long is contributing to the problem.  Maybe the commutators in the motors have oxidized?

What is the standard cleaning protocol for a locomotive that hasn't been run in over a decade?

I'm going to change the harnesses first, but if that doesn't do anything I might try to take apart one of the motors and have a look-see.  

OK, well clearly I'd misunderstood how the control circuitry was set up.  All of the motors are powered through the control board in the A-unit.  The circuit board in the B-unit only controls the horn.  That explains why only the A-unit was getting hot; it was the control circuitry, not the motors, that were heating up.

So in that harness, the brown and blue wires are the power wires for the B-unit motors.  As long as the B-unit is tethered to the A-unit - and the A-unit is receiving power - you can lift the B-unit off the track and its wheels will still turn.

I did switch the harness to the configuration I described above, and it does appear to run more smoothly and consistently.  Coupling the power and ground connectors between both engines allows the control circuitry in the A-unit to take advantage of the additional power wipers/ground contacts from the B-unit trucks, doubling the track-train electrical contact points.  The tracks on our Christmas layout are old, worn, and far from ideal.  Whereas before you'd get occasional cockpit light dimming and small power loss blips, now they remain constant.

I also compared the AB pair to the A unit alone, and it's a definite, noticeable improvement in performance.  So if there was a problem before, it's a least mostly gone now.

For the sake of honesty, I should point out that I had also gone over the trucks and wipers with CDC QD electrical contact cleaner when I had it apart.  I had already cleaned those with IPA about 2 weeks ago, but I wanted to be as thorough as possible, and that may have affected the performance.

One issue I'd like to correct, though, is that removing the pair of brown/blue motor wires has doubled the resistance between the two cars.  The data sheet for the Molex crimps this harness uses states that they will accept up to 22 AWG wire.  Replacing the original 26AWG harness wires with 22 AWG will more than halve the resistance, giving a lower voltage drop between the cars than it had with the original harness. Then again, it might not be worth the effort over < 0.05 V...

I also never got around to taking apart the motors.  I'd still like to clean the commutators, etc, whenever I get a chance.  

I started having another problem, though.  The train now likes to derail when going around turns.  I'd already added a spring to the front truck on each car, like the forums suggested, so I'm not sure what's causing it.  I didn't put a nylon washer between the bottom of the train and the spring, like some posters suggested.  That could be part of it, but I'm not sure.

Any ideas on what might be wrong?