It is possible to replace the brushes without desoldering. Take off the top cap (gently so that the spring doesn't spring out). Undo the two small screws at the rear of the loco. Rotate the motor so that the green wired cap is accessible. Remove the cap slowly so that the spring doesn't spring out, and tap out the old brush. Fit the new brush and spring, and refit the green wired endcap. Rotate the motor back into position, taking care to allow the spring tag to ease into contact with the chassis. Then fit the new brush and spring under the red wired cap. Refit the screws to hold the motor in place.