Before being sworn into state service for the "emergency", the 7th Pennsylvania Emergency Militia had been known as the 3rd Regiment Infantry Gray Reserves from Philadelphia. With hostilities growing in 1861, the Grey Reserves had been organized, and made up of National Guard regiments from the city. The National Guard was not what we think of today, however, it was a well trained and well equipped militia unit that could be called upon by State and Federal government. Many such units provided the back bone of the famed Pennsylvania Reserves, which were originally intended to act as Pennsylvania's last line of defense at the outbreak of the Civil War, before they were sent to Washington to strengthen the Union ranks after the disaster at the Battle of 1st Bull Run. Upon the reorganization, the 3rd Regiment was designated as the "1st Regiment of the Brigade" (the Gray Reserves), and was then referred to as the 1st Regiment National Guard of Pennsylvania.
The 7th PA Emergency Militia was among the very few military organizations that were accepted into state service as an already organized regiment. Generally, after Gov. Curtin's call of Sept. 4, militia companies arrived in Harrisburg, organized into Regiments, and then sent south toward Maryland. Being allowed to keep their regimental structure allowed the 7th to be quickly transported to Harrisburg, and then quickly transferred down the Cumberland Valley Railroad to Hagerstown. There was some displeasure at losing their familiar Regimental designation (1st Regiment), and being given a new one (7th PA Emergency Militia). However, with their presence in the Emergency Militia, Pennsylvania was assured of at least one unit with solid military training and cohesion.
We pick up the 7th's journey on late on the night of September 16th:
The arrival [in Chambersburg on the 16th] was after dark, and the troops were quartered through the night in churches and school-houses, until the next morning when they moved out to a wood on the south side of the town to an encampment known as "Camp McClure."
Instead of a camp, it was scarcely a halt. Orders immediately followed to re-entrain, and the regiment was again on its way, this time over the State line to Hagerstown, Maryland. On the route an issue of ball cartridge was made, forty rounds for the cartridge-box and twenty for the pocket. At eight o'clock on the evening of the seventeenth on its arrival at Hagerstown the regiment left the cars, stacked arms in the main street, and awaited the distribution of what proved to be a very limited supply of rations. The commissariat, by those who looked to it to be fed, was pronounced a failure, and what the soldier got he had either brought with him or gathered up from his own pursuit of it of obtained it through purchase by his officers while on the move. Coffee was a negligible quantity. There was mischief somewhere - nobody cared to inquire where. It was said there were ample stores at the depots, but supplied and consumers rarely met.
Knapsacks and baggage were left behind at Hagerstown, and with lightened load the regiment pulled out for its first real march to Boonsboro. The distance was ten miles, which with an hour's halt at Funkstown was covered before daylight on the morning of the eighteenth. "I remember," reads a note made of the occasion, "that weary march, and how we dropped like logs, in bivouac, at three o'clock in the morning, feeling the coming day might be fatal to some of us; for signs of war and battle were in the air, and the guns of Antietam had been making unwonted music to our ears. Signals on the mountain tops, orderlies dashing by, broken caissons and vacated rebel camping grounds told us we stood on sacred soil; but the battle was over when we reached Boonsboro."
The march [to Boonsboro] was well along, when the regiment pulled out of the road and into the timber for a short halt and a brief rest. Overstrained to the limit of endurance, the men were soon asleep. Other troops began to pass along the road, and their tramp aroused some of the more restless. One especially, bewildered at his sudden awakening, hurriedly gathered accoutrements, knapsack, and musket, and hastened to join the ranks of the moving column, thinking it his own, with the very natural inquiry for his own Company D. "Yonder on the right," was the prompt response. Our new recruit pushed along until he dropped into what he supposed was his place or very near it, neither he in the darkness recognizing any of the men about him, nor they him. He had failed to extend his inquiry beyond the letter of his company. What regiment it was had altogether escaped him. By and by day began to break, strange faces were all about him, and the distant boom of the cannon indicated a near approach to a battle-field. Suddenly it dawned upon him he had forgotten to ask for the regiment, and when he did, back came the answer, "Twenty-ninth Pennsylvania." Without disclosing his mistake, he quietly let himself drift to the rear, and after some tribulation, much fatigue, and a bit of chagrin found his way back to his command again.
Latta, James. History of the First Regiment Infantry National Guard of Pennsylvania (Gray Reserves) 1861-1911. Philadelphia: J.B. Lippincott Co., 1912.