Pontoon bridges have been used since time immemorial, and it is said that the original London Bridge was a pontoon bridge built by the Romans. Continuing this tradition, the Union erected a pontoon bridge of their own at the base of the Harper's Ferry Peninsula, using an opening in the Armory grounds' river wall to connect both sides. The fiery Colonel Benjamin "Grimes" Davis convinced the aging General Miles to allow 1,300 cavalry of the besieged Union forces to try and escape using the pontoon bridge through Confederate lines during the night of September 14th. He was successful, taking out or avoiding the Rebel pickets, the lines of which had been thinned due to the exposure of the flank at Crampton's Gap. Davis also managed to use his native Mississippian accent to convince the head of a ninety-one long Confederate ordinance wagon train to take his forces as escorts. It wasn't until morning that the wagon drivers were surprised that their escorts had pistols trained on each one of them!
Below is a screenshot of a "what if?" scenario (direct CSA attack on the town itself).