Trivia: The soldiers guarding the X-Men on the train are part of the "Mutant Containment Unit", and have "MCU" patches on their arms. Director Simon Kinberg has said while the name wasn't initially intended to connect to the Marvel Cinematic Universe, he of course recognised the significance of the acronym as soon as the wording was suggested and was happy to make the patches quite prominent as a deliberate nod to the X-Men returning to their parent company.
Trivia: Early on in production, the film was actually meant to be an epic two-part story. Part one would be called "Phoenix," while part two would be called "Dark Phoenix." The studio eventually pulled the plug on the second film for unknown reasons, forcing writer/director Simon Kinberg to fold the scripts together into one single film. Many elements of the story fell by the wayside when this happened since they couldn't be fit into one single film.
Trivia: This movie was originally meant to be the start of a new trilogy of "X-Men" movies that would follow the continued exploits of the younger "First Class" X-Men cast during the 90's. Characters such as Charles Xavier, Storm, Cyclops and Jean Gray/Phoenix were expected to return. However, after the merger of Disney and 20th Century Fox, future films were shelved and this movie was slightly retooled to serve as an impromptu finale for the franchise.
Trivia: "Dark Phoenix" is set to be the final main-series entry in Fox's "X-Men" film series. (The spin-off film "The New Mutants" will be the last official entry in the saga, though it will be a stand-alone story with little connection to the main series.) Disney acquired Fox's movie rights during post-production, and after this film's release, the X-Men are planned to be rebooted as part of the Marvel Cinematic Universe.