The Meade’s Line of Battle marker is on the north side of Mansfield Avenue at Hagerstown Pike. (Mansfield Avenue tour map)

From the marker:

U.S.A.
Union Line of Battle
Meade’s Division,
First Army Corps,
Pennsylvania Reserves
Evening of September 16, 1862

This was the starting position the evening before the battle for Meade’s Pennsylvania Reserves Division, who began the fighting on the morning of September 17 with an attack south along the Hagerstown Pike.