The railroad tycoon who is using the gang as muscle is the Big Bad. If it is a villain, though, it should be identified correctly the badass leader of the outlaw gang that causes the most personal trouble is not the Big Bad. In fact, it doesn't have to be a villain at all, as we just said.
This trope is not a catch-all term for the biggest, ugliest villain of any given story. Known as the Shadow in The Hero's Journey. In a serial story, the Big Bad exerts an effect across a number of episodes, even an entire season. A Big Bad could be a character with Evil Plans or it could be an omnipresent situation, such as a comet heading towards the Earth.
The cause of all bad happenings in a story.
