Definitely sounds like a foreign object got caught between the gears. I used to run my R27 set around the holidays on a layout that used the white cotton mats (available at craft stores) for a snowy base. Needless to say the fluffy cotton would shed and was ingested by the power unit's gears. If caught in time I was able to dislodge the jam by reversing directions, but in most cases I had to take the unit apart and clear the jam manually.
-John