in 1861, Manassas Junction emerged as the major railroad depot for supplying the Confederate Army in Northern Virginia
Source: Library of Congress, Route from Manassas to Centreville, August 28th to 31st (by Robert Knox Sneden, 1862-1865)
The Civil War is often described as the first railroad war, starting with the Confederate decision to fortify Manassas and the Union decision to march "on to Richmond" through Manassas (rather than marching due south from Washington DC down the modern I-95 corridor). The Confederates established their line of defense in northern Virginia at Manassas. The Orange and Alexandria (O&A) railroad could bring supplies and troops to that point from the Deep South via Lynchburg, or from Richmond via the connection with the Virginia Central railroad at Gordonsville.
In addition, the Manassas Gap Railroad connected through the Blue Ridge, via Thoroughfare Gap and Manassas Gap, to the Shenandoah Valley. The railroad allowed the forces concentrated at Manassas to provide reinforcements if the Union moved south in the Shenandoah Valley.
General Hooker's army marches past Manassas Junction, June 1863
Source: Frank Leslie's Illustrated History of the Civil War (p.471)
In the end, Confederate reinforcements went the other direction in July, 1861. Confederates fooled Union General Patterson into thinking nothing was happening near Winchester, then marched across the Blue Ridge at Ashby Gap (modern Route 50). Trains carried General Joseph E. Johnston's Army of the Shenandoah infantry from Piedmont Station (modern Delaplane in Fauquier County) to Manassas, where the reinforcements played a decisive role in the Confederate victory.1
The region around Manassas could not provide enough food for the Confederate troops concentrated there. The Orange and Alexandria connected at Gordonsville to the Virginia Central, which ran to Richmond. Supplies from south of the James River came to Manassas via those two railrods.
The Manassas Gap railroad allowed Confederates to ship supplies from the Shenandoah Valley, which became the "Breadbasket of the Confederacy" due to the high productivity of its wheat farms. The railroad shipped so much beef to supply the army at Manassas that a later historian labelled it the "Meat Line of the Confederacy."2
the Manassas Gap Railroad crossed the Blue Ridge, from Thoroughfare Gap on the eastern flank (shown here) to Manassas Gap on the western edge
Source: Frank Leslie's Illustrated History of the Civil War (p.283)
The Confederate army's 1861-62 winter encampment was at Centreville. The Centreville Military Railroad was built from Manassas, across Bull Run, to get supplies to the troops stationed in the front line fortifications.
After the Confederates withdrew from their defense line at Manasssas in March, 1862, the massive meat storage depot at Thoroughfare Gap was burned. The Union army repaired the railroad line and supplied troops that occupied the area throughout the summer, especially forces based on the Rapidan and Rappahannock rivers. After the Peninsula Campaign, General Lee moved the Confederate army westward past the Union army, passing through Thoroughfare Gap and burning the supply depot at Manassas.
the Orange and Alexandria Railroad bridge across Bull Run was rebuilt by the Union
Source: Bull Run Civil War Round Table, The Sketch Artist of the McLean's Ford Redoubt (August/September, 2017)
The Second Battle of Manassas soon followed. Troop movements by railroad were not asignificant element in that fight, but the unfinished line of the Manassas Gap railroad became a focal point of the battle. Stonewall Jackson's troops held the line there, and used the raised bed at Deep Cut for protection. Jackson's resistance along the "Independent Line" of the railroad provided time for General Longstreet to organize his army south of the Alexandria-Warrenton Turnpike, then attack and win a major Confederate victory on August 30, 1862.
the unfinished line of the Manassas Gap railroad was a key topographical feature used by Stonewall Jackson's forces to align troops and fight off Union attacks during the Second Battle of Manassas on September 29-30, 1862
Source: Library of Congress, Atlas of the War of the Rebellion, Battle fields of Bull Run and Manassas, Va. (1892)
The Orange and Alexandria became a key transportation tool for Union armies that were based in Fauquier and Culpeper counties during the winters on 1862-63 and 1863-64, but the Manassas Gap railroad was of minimal military value to either side until October 1864.
the gap in railroad connections between Winchester-Strasburg, and partisan raids on the Manassas Gap and Orange and Alexandria railroads, blocked the Union from occupying most of the Shenandoah Valley except for short periods of time
Source: Library of Congress, Map showing the Fredericksburg & Gordonsville Rail Road of Virginia (1869)
Civil War soldiers blocked rail transport by ripping up rails, then using ties and trees to heat and bend the rails to block quick reinstallation
Source: Library of Congress, Tracks of the Orange & Alexandria Railroad, destroyed by the Confederates between Bristow Station and the Rappahannock (1863)
the Confederates built a military railroad in 1861 from Manassas to Centreville
Source: National Park Service, Manassas Junction and Vicinity
in August, 1862, Union forces marched via what is now Rixlew Lane (red line) betwween modern Wellington and Sudley roads to the Second Manassas battle
Source: National Archives, Map of the Battlegrounds in the Vicinity of Groveton near Manassas
a month before the First Battle of Manassas, Confederates ambushed an Alexandria, Loudon and Hampshire Railroad train loaded with Federal soldiers at Vienna
Source: Frank Leslie's Illustrated History of the Civil War, General Schenk, With Four Companies of the First Ohio Regiment, Surprised and Fired Into By a Confederate Masked Battery Near Vienna, VA June 17th 1861 (p.29)
in the winter of 1861-62, Confederates built a military railroad to support the camp at Centreville
Source: Library of Congress, New defenses erected at Centreville, Virginia (1863) by Robert Knox Sneden
railroads in Northern Virginia, 1852
Source: Library of Congress, Map of the Virginia Central Rail Road showing the connection between tide water Virginia, and the Ohio River at Big Sandy, Guyandotte and Point Pleasant