I received the following from a CW historian. It all seems pretty acurate to me. I have no list of possible candidates as to who it could be - YET! I was saving that daunting task for when I had a time-line to go by. Thanks one and all for your continued help!
1. He wears a sword, not a saber. Swords have straight blades, designed for thrusting. Sabers have curved blades, designed for hacking from horseback at a standing infantryman (or, aka Johnny Depp, pirate). It appears to be a model 1860 Staff & Field, worn only by majors, colonels or generals, or any lower grade commissioned officer on any of their staffs. This model was regulation from 1860 up to World War 1.
2. The soldier wears a frock coat, with shoulder insignia. The rank on the shoulder insignia is in the center of the strap, so his rank is higher than lieutenant or captain.
3. a photo 4 x 2 inches probably was a cut-down CDV, Carte de Visite - french for visiting card. They measured 4 1-4 x 2 1-2. These were common 1856 to 1890.
NOTE: He is mistaken on the size of the original photo - it measures 24 inches by 18 inches not the 2'x4'('=feet) that I had originally posted.
4. You haven't seen many photos of soldiers with swords or sabers for one primary reason; 75% of the army was infantry, and only infantry officers and sergeants had edged weapons. Most earlywar cavalrymen had sabers, but they comprised only about 15% of the Army. Artillery officers had sabers, but the enlisted artillerymen had none, or at best, had the model 1832 short sword, which was patterned after the Roman gladius. Artillery about 8% of the force. Within the remaining 2 %, few had blades.
5.As for this person serving the Engineers, I don't think so. The insignia on the front of his kepi might be construed to be the castle emblem of that service, but I believe the insignia is a wreath surrounding letters or numbers. If letters, it might be NY for New York, or MS for Medical Service, etc. etc. If numbers, it would be the regiment he was assigned to. I think I can see two letters, but can't be sure.
6. Most important of all, this is NOT a photo of a Civil War soldier. Based on at least three things, this photo was taken during the Indian War era.
A. the kepi has a very low front, which was reduced in height from the CW pattern in the Uniform Revision of 1872. Also, the wreath style insignia became regulation at that time (it was quite uncommon during the CW).
B. The collar height of the frock coat is very low, which also changed with the M1872 uniform.
C. His belt plate appears to be attached to the backside of the plate. A wire loop was soldered to the backside of the plate with the M1872 revision.. The CW version, the model 1851, had the belt loop cast as part of the plate adjoining the right edge of the plate.(to his right side). The uniform in this photo would have been regulation from 1872 to about 1890, possibly as late as 1895, when the uniforms changed just before the Span-Am War.