I don't specifically know where he was in 1860, but I do have the following information:
Charles Fenton Mercer Garnett was born 7 October 1810 at a the Garnett family home called "Elmwood" in Loretto, Essex County Virginia. He died 6 March 1886 in Norfolk, Virginia. Son of James Mercer GARNETT and Mary Eleanor Dick MERCER of Essex County, Virginia.
On January 1, 1842, former U. S. Senator and Georgia governor Wilson Lumpkin, retired from public service, reluctantly took over as disbursing agent for the railroad. He began pushing to resume work and found Charles Fenton Mercer Garnett, whom he put into Stephen Long's old position of Chief Engineer. Garnett's first act was to establish his office in Whitehall, near the southern terminus of the railroad. One of Garnett's later acts was to propose the name Marthasville in honor of Lumpkin's daughter, Martha Atalanta Lumpkin. SOURCE: ngeorgia.com - "Creaton of the Western & Atlantic RR" Marietta, GA history: By 1838 roadbed and trestles had been built north of the city. Construction continued until 1840 when Long quit, having been criticized by politicians for being too slow. He felt the criticism unfounded and he was probably correct. For two years work came to a standstill until another engineer was found. On February 7, 1842 Charles Fenton Mercer Garnett took over, using the area that would become Atlanta as his base.
As crews began to clear and grade north of the town a new pastime became popular. The roadbed was perfect for horse racing, and the sport grew quite popular, taking place in the approximate area of the present-day Welcome Center. The Western and Atlantic began to operate from Atlanta to Adairsville in 1845 and through to Chattanooga in 1850.
Charles Fenton Mercer GARNETT was working in 1850 as the chief engineer of the Virginia and Tennessee railroad at which time he was chosen as chief engineer of the Memphis & Charleston Railroad Company [selected for the job on April 29, 1850]. By the end of the year he completed a preliminary reconnaissance of the route of the Memphis & Charleston Railroad line and announced it at a meeting in Tuscumbia, Alabama. The present site of the public railway depot [in Atlanta] was in fact chosen by Charles Fenton Mercer GARNETT.
In 1856, C. F. M. GARNETT left the United States to take up new engineering challenges in the jungles of Brazil. From 1856 to 1859 he was the chief engineer of the Dom Pedro II Railroad, the first railway line in Brazil. Charles Fenton Mercer GARNETT never married and on March 6, 1886 he died at Norfolk, Virginia. He left no children or direct descendants. [Source: Garnett Family Registy.]
One other note: One of the MARTA transit stations in Atlanta is the Garnett station, named for him.