... is given a dispatch dated "Opposite Fredericksburg, Dec. 11, 3 P. M." Between 4 and 5 o'clock this morning the pontoon train in charge of the 17th and 50th New York Engineers, under command of Gen. Woodbury , proceeded to the river bank, where, with infantry supports, an attempt was made to throw three bridges across the river--one at the point where the railroad bridge formerly crossed, and two more opp...