Had the same problem. Noticed loose wires when i removed the body.
The train only derailed in one direction. All the time, the front motor was dragging the rear with it.

The electrical wires were too short, and stressed when turned the bogies. 

I also rounded the edges of the plastic bogies, which made contact with the chassis in these sharp curves. Solving the narrowness problem.

When you remove the body, review the four electrical wires to the wheels.
Probably they are too short, and are stressed when turning the bogies. 
The bogies can pull these wires loose from the contacts after making sharp curves.

The train might not work at all, or will derail when only one motor is operating, so one might think it's a rail problem.

When you remove the body, review the four electrical wires (too short) to the wheels.
The bogies can pull these wires loose from the contacts after making sharp curves.

The train will derail when only one motor is operating, so one might think it's a rail problem.

I also rounded the edges of the plastic bogies, which made contact with the chassis in these sharp curves. Solving the narrowness problem.