According to Word of God, Tulip is intended to be a foil to the Infinity Train itself. The Infinity Train is bizarre and illogical, which contrasts with Tulip's need for everything to be logical and make sense.
Tulip's also one for the Conductor, since both are Control Freaks in a sense, who want the world to do their bidding. The main difference is that Tulip understands and eventually accepts that life can't always go her way. She also cares about the friends she makes on her journey while the Conductor sees the character on the train as puppets who have to stay in their cars for all eternity. If they rebel, she'll turn them into Ghoms without hesitation, in direct contrast to Tulip, who encourages her to explore. Just like Tulip, the Conductor/Amelia was also a person who entered the train, but while Tulip was able to make friends and lower her number by resolving her issues, Amelia just became more and more obsessed with what she had lost, thus increasing her number to an amount that has crawled up to her neck and eventually taking control of the train itself in a desperate attempt to recreate her old life and her lost love in one of its cars, which Tulip can reject when Amelia offers to do the same for Tulip, creating a world where her parents were still together because she knew that it wouldn't be real.
Jesse Cosay is the complete opposite of Tulip Olsen in many, many ways.
He hails from Arizona (given the "A" on his jacket and patch that represents the Arizona flag) as a Native American from the Apache tribe in comparison to the Caucasian Tulip hailing from Minnesota.
His sidekick is a deer (who he named himself) compared to Tulip and One-One the robot (who named themselves). He also seems to enjoy singing while Tulip was embarrassed by it.
His problems stem from him being pleasant to the point of being unwilling to displease others, whereas Tulip's issue was going through her parents' divorce and being unable to understand other people's feelings. Also, his number is significantly lower than Tulip's (initially at 31, Tulip was at 115) with the number change the first time it happens to be the opposite (Tulip's went down when she mentioned a memory with her father, while Jesse's rose because of him being a bit of a doormat).
He's on the swim team at school while Tulip was known to be in academics.
Tulip is an only child and is known to have one supportive friend in Mikayla, while Jesse has numerous "friends" who take advantage of his kindness and use peer pressure to bully his little brother.
He's fond of naming animals after Dracula, whereas Tulip's original surname was Van Helsing — Dracula's nemesis.
It's very telling that The Cat has different reactions to Grace and the Conductor. The Cat is terrified of the Conductor and worked with her out of self-defense (but had safety measures to bite her in the future). As for Grace, The Cat hisses at her and wants that bitch out of her sight.
Grace is also this to One-One, the true conductor, in terms of their Lack of Empathy towards the train's denizens and belief they are supposed to help passengers. Grace's idea of their help involves rampaging their cars while One-One simply doesn't understand if they would want to leave the train.
Grace is also the opposite of Tulip: Tulip tries to abide by the rules of the Train (initially believing that her number going down is a bad thing but later accepts that it going down is okay), learns to accept the train as a learning experience and makes friends with the denizens of the train (especially The Cat) while Grace would rather the train goes and fucks itself and has an escalating number on her arm because she doesn't care about changing herself and has animosity with The Cat. Grace has a dancing background and her problems stem from having no friends and very distant parents while Tulip is into coding and games and her problems are related to her parents' divorce.
Grace's also this to Jesse. Jesse wants to make everyone his friend, and Grace wants to make everyone her follower. Jesse initially had trouble picking a side, but Grace is not afraid to take action. Jesse suffered from a bad influence, and Grace is the bad influence. Jesse sees the train as something to explore, and Grace sees it as something to plunder. Jesse was quick to make friends with MT, Grace didn't warm up to Hazel until halfway through Book 3.
Book 3 makes Simon a contrast to Tulip, surprisingly. Both of them are obsessed with rules — Tulip with rules in how to survive the train so she's not "gone forever" and Simon with procedures to deal with Nulls — but Tulip learned to be much more flexible, learns to see the denizens as her friends and see her number dropping as an okay thing, Simon is quite rigid in his thinking and will even go as far as to wheel Nulls if he feels that they hinder "progress" and denies the idea of even wanting to go home and improving himself.
MT's a mirror for both regular Tulip and Jesse.
MT to Tulip: Outside of being her reflection, she essentially acts just like Tulip originally did when she ventured through the cars, not wanting to deal with anyone and wanting to be on her own. The major difference is that Tulip was acting like that under the major assumption that her number going down meant being "gone forever", whereas MT has to run away or else the Mirror Police find her and grind her to dust. Her initial reaction to the number going down is for selfish reasons: Tulip earnestly wants to go back home, and MT just wants to get the hell out of Dodge.
MT to Jesse: Jesse is easygoing and would rather talk things out instead of going through brute force. He also is unwilling to displease anyone while MT is more honest about how her opinions. Jesse’s willingness to let other people’s decisions control him makes him metaphorically a reflection—a lifestyle MT has already rejected.
The Conductor/Amelia to One-One. When she confronts Tulip in "The Ball Pit Car", she talks down to Tulip and blames the girl for getting her friends into trouble and mockingly tells her "No more tears" when Tulip is crying. One-One on the other hand, comforts Tulip and tells her it's okay to cry. Moreover, it's the human who is telling Tulip not to show emotions whereas the robot is consoling her.
Grace's parents to Tulip's parents. Tulip's parents were divorced and going through a lot of issues but still loved their daughter, while Grace's parents are married and would rather shove her off to a caretaker than deal with her.
