Like Moorcock's Warlord of the Air, Harrison's A Transatlantic Tunnel, Hurrah! takes place on a world parallel to our own. On Harrison's fictional world, the American Revolution was halted in its infancy and George Washington executed as a traitor to the Empire. However, where many writers would mark that as the point of divergence between our world and the world of the story, Harrison's divergence point occurs several centuries earlier than the late 1700s.
Harrison postulates that Spain never emerged as a world power because the Moors were never booted from the Iberian peninsula. He accomplishes this by having the Moors win the Battle of Navas de Tolosa in 1212. Because of this, the Catholic states in Spain never allied under Aragon and Castille. This then extended to Columbus never getting his funding for the cross-Atlantic voyage in 1492 and left the way open for John Cabot to discover America instead.
This would eventually translate into Britain dominating both North and South America, and creating a worldwide empire of such power and magnitude that it could do pretty much whatever it wanted. From Harrison's 1976 article explaining the genesis of the story:
"Now the idea of the transatlantic tunnel became exciting -- and possible. With the English explorers opening up all of South, Central and North America, as well as India and all the rest, the power of the empire would have been incredible. The African colonies of the other European colonists could be picked off one by one if needs be. If the European countries united early enough they might have stopped the growing British strength, but in my book they never got around to it. Divide and rule is the name of the game, so the European states still exist and monarchy is the rule, with all the royal families united -- as they once were -- and the power of Britain behind each one if needs be."With this newfound strength on the part of Britain, can you see why the American Revolution failed so utterly in Harrison's world? Which of course immediately brings up the hero of the novel -- Augustine Washington, the descendant of George, who wants to clear his ancestor's name. Gus works with Sir Isambard Brassey-Brune, the descendant of Isambard Kingdom Brunel, on the project of building a Transatlantic tunnel between the heart of the Empire and its far-flung North American colonies.
The story is Neo-Victorian in that it takes Victorian ideals and technologies and places them squarely in the 1970s. Harrison discovered the novel had to be this way based on research he did over the course of a five-year period. He also figured out that he couldn't write a Victorian novel straight, and thus had to make it either humorous or at the very least a parody. Except ... according to the people in the fictional world, the novel would be taken at face value. We, on the other hand, would laugh at the differences -- the burned-out Mount Vernon and the paltry Washington house next to it, secondary characters such as detective Richard Tracy and the minor Royal British Intelligence executive J.E. Hoover.
Through the course of the story, we see coal-powered flying machines among the vast array of late Victorian high technology that makes up part of the Steampunk aesthetic. In addition, we also have a classic tale of engineering that fits quite nicely in with the science aspect of Steampunk novels. As a result of this, and the language of the novel, Harrison fits into the proto-Steampunk canon quite nicely.