My theory is multiple drivers are rarely in perfect sync. That causes the engines to fight each other. Lionel gets arounf that problem with back drivable gears but a lot of earlier engines don’t have those mechanics. With one driver all the motors get the same voltage applied.
My six motor UP E8s only draws 1.4 amps running light. Of course total stall current is about 13 amps so protection is needed.
Pete
My thinking was similar. But DC can motors are natural transmissions. If one was driven faster (higher voltage) than another, the higher voltage motor would just bear more of the load. Thinking a little further, and, assuming the electronics provide pulse width modulation to control power, one could be on while the other is off - out of sync. But I'd think the mechanical inertia of the drives would absorb that. Can the modules detect overspeed and reverse polarity to slow and engine running faster than commanded? That would explain it.
What's the definition of "back drivable gears". The ability of a worm drive to be rotated by the wheels?
And why is the rectifier the weak link? Non-optimum part for the job. I would think the drive IC's would be the weaker link. Or is it just the lack of heat sinking where the drive ICs have some.
All academic on my part as you have the solution. Just very curious on the inner workings of the system, you know, a little kid's "why"

. Sorry to belabor the issue.