WPA projects would often shun machinery and heavy equipment, so that lots of unemployed laborers could find jobs at a particular work site. In other words, better to have 50 men with picks & shovels than 10 men and two tractors. While not the most efficient way to complete a project, it did bring paychecks, hope, and a sense of belonging to more unemployed workers.
Still, there were many times when machinery and tractors were used. For example, perhaps there were not enough unemployed workers to get the job done in a reasonable time. Perhaps one of the unemployed workers had experience with heavy equipment and the foreman was able to secure funds to rent a tractor. Perhaps private contractors were working on part of the project. Each WPA project had its own "personality" and thus the construction methods and strategies varied.
Take a look at some of the interesting machinery used at WPA work sites around Maryland, between 1935 and 1943. All photos courtesy of the University of Maryland College Park Archives.