To an unaccustomed observer the concourse of the trains of an army, in connection with any rapid movement, would give the idea of inextricable confusion. It is of course necessary to move them upon as many different roads as possible, but it will frequently happen that they must be concentrated in a small space, and move in a small number of columns. During the celebrated 'change of base' from Richmond to Harrison's Landing, the trains were at first obliged to move upon only one road--across White Oak Swamp--which happened fortunately to be wide enough for three wagons to go abreast. There were perhaps twenty-five hundred vehicles, which would make a continuous line of some forty or fifty miles. While the slow and toilsome course of this cumbrous column was proceeding, the troops were obliged to remain in the rear and fight the battles of Savage Station and White Oak Swamp for its protection. A similar situation of trains occurred last fall when General Meade retired from the Rappahannock, but fortunately the country presented several practicable routes. It is on a retreat, particularly, that the difficulty of moving trains is experienced, and thousands of lives and much valuable material have been lost by the neglect of commanding officers to place them sufficiently far in the rear during a battle, so as to permit the troops to fall back when necessary, without interruption. A march being ordered, supplies according to the capacity of the trains, are directed to be carried. The present capacity of the trams of the Army of the Potomac is ten days' subsistence and forage, and sixty rounds of small-arm ammunition--the men carrying in addition a number of days' rations, and a number of rounds, upon their persons. When the wagons reach camp each evening, such supplies as have been expended are replenished from them. As a general rule the baggage wagons camp every night with the troops, but the exigencies are sometimes such that officers are compelled to deny themselves for one or even two weeks the luxury of a change of clothing--the wagons not reaching camp, perhaps, till after midnight, and the troops resuming their march an hour or two afterward. Those who indulge in satires upon the wearers of shoulder straps would be likely to form a more correct judgment of an officer's position and its attendant hardships, could they see him at the close of a fortnight's campaign. Like the soldier, he can rely on nothing for food or clothing except what is carried by himself, unless he maintains a servant, and the latter will find a few blankets, a coffee pot, some crackers, meat, sugar, coffee, etc., for his own and his employer's consumption, a sufficient burden. Let us see how the supplies of the quartermaster's department are distributed.