How did the advantages of the north at he start of the war continue to be advantages?

The chief advantage was simply a much bigger population from which to recruit armies. When Grant became General-in-Chief, he ended the system of prisoner-exchange, knowing that the Confederates would run out of men first. So that advantage steadily mounted-up.

Another was the much greater railroad mileage in the North, and the capacity to move troops and supplies by rail. Again, as the Union armies began to destroy Southern railroads, the disparity in rail mileage widened further.