On July 9, 1864, at 2:00 a.m., the last train of Union reinforcements pulled into Monocacy Junction just hours before the battle. The addition of 3,400 veterans increased Wallace's total to 6,600 men. While Wallace was successful in delaying the Confederate march on Washington, he failed to stop the Confederates from destroying the junction and damaging the railroad bridge.
Railroads played a major part in the Civil War. The ability to move troops and supplies rapidly over short and long distances had an immediate effect on campaigns and battles. Because of this advantage, railroads would become prime targets for both armies throughout the war.
A large force of veterans have arrived by water, and will be sent immediately [by train]. John W. Garrett, B&O Railroad President