Unit 1 was traveling up river on the Ashley River. Unit 1 had been stopped by Coast Guard Auxillary and had apparently left the stop before it was completed. When unit 1 approached the CSX railroad tressle by Drayton Hall the draw bridge was down. The operator of unit 1 tried to drive unit 1 between the pillings of the tressel. Unit 1 became wedged between the pilling of the tressel.