Amfleet said:
Athearn models can run inconsistant. I have 3 P-42 units (all Phase IV), each one is either slightly higher or lower than the other in height and the pulling power varies. I'm surprised though that your locomotive feels light? The only thing I can suggest is taking the tops off both units and comparing the motors to see if there are any differences. If there are report it to Athearn right away, they may suggest making some slight ajustments to the motor or send you a whole under body.
Well when I took the body off both units I found why. Quite Simply, the chassis, so to speak, is made of what seems like much lighter metal. It is even the color (the grayish color) of the engine itself.