Parsons worked for the
New York, Lake Erie and Western Railroad from 1882 through 1885. He wrote
Turnouts; Exact Formulae for Their Determination (1884) and
Track, A Complete Manual of Maintenance of Way (1886) which both addressed railroad problems, and this interest in rail transportation continued throughout his life. and was responsible for the construction of the
Interborough Rapid Transit (IRT) subway line. He left New York in October 1886 to serve as Chief Engineer for the
Fort Worth and Rio Grande Railroad, although he retained his affiliation with the District Railway Company. In 1887, he became the Chief Engineer and General Manager of the Denver Railroad and Land and Coal Company. He returned to New York in 1891 upon the completion of these railway projects and a number of water-work ventures in Mississippi. Parsons was appointed to the
Isthmian Canal Commission in 1904 by President
Theodore Roosevelt. In early 1905, he traveled to
Panama as a member of the committee of engineers which favored a sea-level canal. Parsons was the Colonel of the 11th Engineers of the
American Expeditionary Forces (AEF) in France during World War I. He was with a team of engineers in
Battle of Cambrai that was suddenly attacked by Germans while making railroad repairs; the engineers fought back with picks and shovels. He was awarded the
Army Distinguished Service Medal for "specially meritorious services" and received decorations from Great Britain, France, Belgium, and the state of New York. The citation for his Army DSM reads: ==Personal life==