Hell on Wheels is an American Western television series about the construction of the First Transcontinental Railroad across the United States. The series, which featured Anson Mount, Colm Meaney, Common, and Dominique McElligott, chronicled the Union Pacific Railroad and its laborers, mercenaries, prostitutes, surveyors, and others who lived, worked, and died in the mobile encampment, called "Hell on Wheels", that followed the railhead west across the Great Plains. In particular, the story focused on Cullen Bohannon (Mount), a former Confederate soldier who, while working as foreman and chief engineer on the railroad, initially attempted to track down the Union soldiers who murdered his wife and young son during the American Civil War. The series, which broadcast in the United States and Canada on the cable channel AMC, aired from November 6, 2011 to July 23, 2016.

What is the series hell on wheels about? Let's have some stream of consciousness first.
To answer the question, consider the following: Hell on Wheels is an American Western television series about the construction of the First Transcontinental Railroad across the United States. So, the answer is the construction of the First Transcontinental Railroad across the United States.