Martin Stein vehemently refuses to board the Titanic, decrying the builder as an idiot. Victor Garber, who plays Stein, played Titanic designer Thomas Andrews in Titanic (1997).
Victor Garber (Professor Stein) and Billy Zane (P.T. Barnum) also starred together in Titanic (1997).
The TV in the background at Nate and Amaya's apartment is showing the events of Finish Line (2017). Specifically, the speed force lightning storm.
One of Barnum's carnies wears the costume of DC superhero B'wana Beast. In some continuities B'wana Beast and Vixen are romantically involved.
PT Barnum refers to Steel as "The Man Of Steel" - a reference to Superman.