I've had similar problems with these 4-4-2s. Personally I think the wiring Lionel MPC used in the motor/chassis is underspec'd for the amount of current they can pull. Note the MPC 4-wheel motor has a larger armature and more laminations than its postwar counterpart. Plus these are carrying the heavy 2037 style cab, but have the taller gearing of a 2034. When you pull a heavy load the rubber tires prevent the wheels from slipping and things can get HOT. This is even more true the chopped-wave power supplies common today.
The advice I got here on the Forum is what Jim told you-- replace the brush springs. I bought new ones, but never got around to installing them 
. Let us know how you make out.
