Are you translating a sentence from English or to English ? !.
"In winter I relied upon my violin, though during the construction of the Troy and Saratoga railroad, I performed many hard days' labor upon it".
The "it" at the end of the sentence is a little unclear. However, logically it should refer to the construction work on the railroad. If so, then the highlighted part means "I spent many days working hard on it (the railroad project)".
Student or Learner