Tulip being somewhere on the autistic spectrum is a common headcanon, due to having a special interest (programming, in her case) and also a lack of understanding of other people's emotions, common symptoms for those on there.
It's never explicitly stated whether Amelia and Alrick were still only engaged or if they had married by the time of the latter's death. Canonically, it's the former, but pretty much every fan depicts them as the latter. Not helped by the fact that the only mention of their relation is when Tulip says "I'm sorry about your husband, Amelia" and Amelia doesn't correct her otherwise.
MT is frequently headcanonned as nonbinary, due to her being a reflection and thus probably lacking a sex, having an arc about being her own person and distinguishing herself from Tulip, her androgynous appearance in Book 2, and her choosing of the name "Lake" (which follows the naming conventions favoured by younger nonbinary people)note That is to say, using place and object nouns rather than traditional names due to the limited pool of gender neutral ones. Whether or not fanworks taking this approach have MT use gender neutral pronouns in addition to (or in place of) the female ones she responds to in canon depends on the writer.
Simon is the only main character to never have his pre-train backstory revealed in any way, outside of once ranting about funerals. That said he is commonly believed to have come from an abusive household, usually from Parental Abandonment or Parental Neglect, given how he displays clear trust and abandonment issues.
Less commonly, Owen Dennis's joke that Simon failed a spelling bee is taken as canon despite Owen Dennis saying it sarcastically and later stating he prefers fans to come up with their own answers for mysteries in the show.
Simon is frequently headcanoned to be Quebecois, due to his rather French-sounding name.
Fans like to speculate that the Ghom that killed Simon was either Tuba's soul turned into a Ghom getting revenge on what he did to her or the same one that traumatized him for life all those years ago.
Though Word of God says that they're both 18, since this is never outright stated in the show, many fans depict the duo as being in their early twenties, with Grace being slightly older than Simon.
In the aftermath of "The New Apex", some fans began theorizing that being devoured by a Ghom causes one to reincarnate as a train denizen. It mostly gained popularity since it would both serve as a Fate Worse than Death for Simon, as well as a chance for him to start over and eventually atone for his past crimes.
The Ghoms are more dangerous to passengers with higher numbers because, when they attacked in the last episode of Book 3, the one that attacked Grace—whose number had been greatly lowered—barely hurt her at all, while the one that attacked Simon—whose number had gone so high it covered his entire body—drained and killed him almost instantly.
After "The Train to Nowhere", a lot of fans ran with the headcanon that Ryan and Min-Gi had started dating by the time their gig in New York is shown, as Ryan's behavior towards Min-Gi on stage can come across as rather flirty and the lighting has very prominent rainbow colors.
