Harper’s Ferry, Oct. 3.—The railroad bridge was completed yesterday, and trains of cars passed over it into Harper’s Ferry to-day, with troops and munitions of war and army stores. The Winchester road is being put in order, and trains will run at once to and beyond Bolivar Heights.

There is much uncertainty here as to the movements of the army. A strong advance will undoubtedly be made into Virginia, but whether it will be by way of Harper’s Ferry or over the Williamsport Ford, no one has the slightest idea. So long as the Confederates lies in his present position it is said no hurry will be evinced on our part to disturb him.—A large body of the Confederates are still at Failing Waters, and at Bunker’s Hill in the rear of Martinsburg, whilst a few regiments are at Charlestown, their pickets meeting those of Genera! Sumner about three miles this side of that place.

How the Disaster Occurred.

The surrender of Harper’s Ferry surprised everybody when it was announced. It was not easy to understand how it had become necessary. Col. Miles had under his command about 13,000 men, including the cavalry that escaped. He had all the artillery necessary to a successful defence of the position, including fifty pieces of field artillery, besides all the heavy guns in the various batteries on the Heights. There was no lack of ammunition or stores, and the place should have been held against double the force that was brought against it. The batteries on Maryland Heights were finely placed and very powerful. This was the key to the position, and a capable and determined officer would have held it success fully without serious difficulty.