Don't know if there is a single event that qualifies. The most significant long term effect on America was the railroad; it affected everything from technological innovation to accounting to the first large scale business management techniques to expansion of territory that could be exploited to interstate commercial relations to making other large interstate and international corporations viable to massive multi-national financing enterprises. I'm certain I've forgotten many other important effects, but it's still early in the 'morning' for me. lol
Most everything that happened after the railroads began expanding is related in some way or other to them, politics, banking, other businesses no longer merely regional enterprises but national ones, like Swift and Armour, Sears Roebuck, Carnegie Steel, Standard Oil, etc., etc,