Atlas O Motor Failure?

Steven: I think your first step here would be to remove the engine from the layout and power it up on a workbench to verify whether the motors are indeed turning the wheel on their respective trucks for starters. Do this by grounding one lead from your power source to the frame in a bare metal area and put the other power feed to one of the center rail pickup rollers. Once you have that information, get back to use here and we'll troubleshoot from there. 

I have never had a motor failure in all my years, although that's not to say you can't have it happen

I've had a couple of motor failures, excepting the cheap ones in starter sets, I've replaced a few of those.  However, for a quality can motor, it's pretty rare for them to fail.

If you want to be sure the motors are working properly, the best way is to disconnect them and individually power them on the bench with a DC power supply.  Failing any other source, an HO power supply is a good source of DC.  Apply some friction to the wheels and see if you detect any issues with power.  It will be fairly obvious if one of them is failing.

I'm a little confused on what you said, you just want me to make sure the motors are turning the axles the way they should be?

Yes Steven; I'm just trying to find out if each truck's wheels turn when you power the motor, or whether one motor is turning and the other one may NOT be turning...

Did you rotate each flywheel by hand and make sure there is no binding in the trucks/motors.

I'm about to go down and test the engine out, but I can tell you for sure there is nothing physically forcing the motors to a stop. The wheels turn smoothly in both directions, and all the gears are perfectly aligned. It's only under load the engine struggles to move.

Well, debugging is simply the process of eliminating possibilities until only one remains.  Once you verify each motor runs properly, the trucks are not binding in any way, you can move on to other possibilities.  You could simply have a poor connection somewhere feeding the electronics and you're getting a power drop under load.

FWIW, I've had more failures with the TAS EOB motor drivers than I ever had with can motors.  I actually got pretty good at replacing the drivers on those, and it's somewhat of a PITA as they appear to have assembled it not to be easily repaired.