This is part of the result of General Pope's too rapid advance to head off
Lee's army south of the Rappahannock River. Although overtaking the
advance of the Confederates at Cedar Mountain, Pope had arrived too late
to close the river passes against them. Meanwhile he had left the Orange &
Alexandria Railroad uncovered, and Jackson pushed a large force under
General Ewell forward across the Bull Run Mountains. On the night of
August 26, 1863, Ewell's forces captured Manassas Junction, while four
miles above the Confederate cavalry fell upon an empty railroad train
returning from the transfer of Federal troops. The train was destroyed.
Here we see how well the work was done.