I think that your locomotive is made like the one in the photo below. There are two small tabs (red circle) on the back of the motor, one on each sideplate. They are bent slightly outward to keep the motor in place. The two tabs need to be bent slightly inward toward the middle of the motor (direction of the green arrow) so that they will clear the opening for the motor. If I remember correctly, this will allow the back of the motor to move away from the body and then the motor can move backward slightly, allowing the slots on the front of the motor to disengage from the body, then tip the front of the motor downward a bit, and slide it out of the body.
