Whatever, again, the condition of the lines when taken over, the railways of both combatants were subjected to constant attack by the other side with a view to the interruption of communications, the destruction of railway track, railway bridges, rolling stock and other railway property being enormous. Reviewing the general situation at this time, McCallum says in his report:-- In the beginning of the war military railroads were an experiment; and though some light as to their management had been gleaned by the operations of 1862 and 1863, yet so little progress had been made that the attempt to supply the army of General Sherman in the field, construct and reconstruct the railroad in its rear, and keep pace with its march, was regarded by those who had the largest experience, and who had become most familiar with the subject, as the greatest experiment of all. The attempt to furnish an army of 100,000 men and 60,000 animals with supplies from a base 360 miles distant by one line of single-track railroad, located almost the entire distance through the country of an active and vindictive enemy, is without precedent in the history of warfare; and to make it successful required an enormous outlay for labour and a vast consumption of material, together with all the forethought, energy, patience and watchfulness of which men are capable. To meet the various conditions which had thus arisen, McCallum was authorised by the Federal Government to create two distinct departments, destined to bring about a still further development in the application of rail-power to war by establishing precedents which the leading countries of the world were afterwards to follow more or less completely, according to their own circumstances and requirements. The departments were known respectively as the "Transportation Department," embracing the operation and maintenance of all the lines brought under use by the army of the North; and the "Construction Corps," which was to repair the damage done by wrecking parties of the enemy, maintain lines of communication, and reconstruct, when necessary, railways captured from the enemy as the Federals advanced. Concerning the Construction Corps, and the great work accomplished by it in keeping the lines open, details will be given in the chapter which follows.