I've always heard that trapdoor rifles chambered for .45-70 were popular toward the end of the Civil War. However, research shows that it was officially adopted for use in the 1873 Springfield rifle, which means well after the war. Was .45-70 ever actually used during the Civil War?

No, the first converted rifles were 50/70 and IIRC that was after the war.

Quoted: No, the first converted rifles were 50/70 and IIRC that was after the war.

.50-70 was adopted in 1866, right after the war.

The first trapdoors were the M1865 "First Model Allen Conversion" in .58 Rimfire, still with 40 inch long barrels.

A farely large number were made, since it was a cheep, easy conversion.  But the Army did not like them and few were ever issued.  However many were sold to the Union Pacific, the largest number of their spent cases can be found along the original trans-continental railroad right of way.

I do not beleve though that any trapdoors were issued before April 1865.  

The M1866 was a much more radical conversion.  Requiring not only adding the trapdoor action block at the breech but also boring out and resleeving the barrel to take it down to .50 cal for the .50/70 centerfire cartridge and then shortening the barrel by 10 inches at the front and resighting.

Most Army units in the west went streight from the rifled muzzle loader to the M1866.

No.
1873 was a very good year, .45-70, and .45 Colt ctc. and pistol.

BTW, the year before Custer and many of his men were wiped out, the 7th Cavalry was equipped with "obsolete" Spencer repeater rifles.