The Infiniy Train may be a representation of Tulip´s mind, thats why she cant escape.
Think of it, One-One may represent two of her main emotions. She is depressed/sad becuase she cant leave the train, or bad thoughts in her mind, if we think of the train as her mind, and she is sometimes happy becuase she gets news about the train or the number.
The dogs, and Atticus, may represent, that maybe, Tulip has a love for animals. Specially dogs.
The spider may represent that she first thought of spiders as monsters, but then she learnt more about them and didnt care.
The Steward may be one of her inner demons.