In Split, we learn that Kevin's father boarded a train and never came home, leaving him with his abusive mother.
In Unbreakable, the incident that leads David to discover his extraordinary strength and impenetrability is... a train crash, set up by Elijah.
Naturally, many fans have assumed there's a connection between the two train-related occurrences, and we absolutely think that will be the case.
All the pieces fit together perfectly. Kevin, toward the end of Split, is seen placing flowers on a train platform. Also, his transformation into The Beast takes place on an Amtrak train - the same type of train that crashes at the start of Unbreakable.
In short, Elijah caused the crash that both revealed David's powers, and killed Kevin's father.
This will turn Kevin on Elijah - after the two team up for the first chunk of the movie - and the breaking up of this alliance is something that needs to happen, because of something Elijah says in Unbreakable.
He theorises that the villain is "the exact opposite of the hero", and this is what brings the two into conflict. Kevin/The Beast can receive a shotgun blast at point-blank range and not sustain any injuries. Physically, he's the polar opposite of the frail Elijah.
So, wouldn't that make Kevin the hero to Elijah's villain, or the villain to his hero, depending on how the character views himself? If the two men are in cohorts at the start of the film, then there must be an incident that brings the two to oppose each other, fulfilling Elijah's 'opposites collide' hypothesis.
And I don't know about you, but if I managed to find the man who killed my father, I'd be pretty mad at them.