What was the major innovation that defined the second industrial revolution?

There are many innovations such as, sewage systems, railroad networks , telegraph, and new innovations in steel production .

I believe the railroad and the invention of the telephone were the most important. The railroad allowed Americans to travel from coast to coast in very little time. The telephone allowed for instant communication as before a letter would take weeks to receive.

Ithink it's  established religious tolerance and influenced the us constitution.