Considering the events of Infinity Train Book 4 happen at the same time as Book 5, I think it's fair to assume they would have formed a diptych of sorts. On one hand, that makes the cancellation of the series even more frustrating. On the other hand, we can make some theories purely based on things that feel like they could be meant as intentional parallels. We only have one half of a conversation, but we can try to guess what's missing.
The most obvious thing is that Min and Ryan knew each other since childhood, like Amelia and Alrick, and seem just as inseparable. They had planned their future lives thinking the other person would be there forever. We already know what that did to Amelia when disaster struck.
We don't know when Amelia found the Engine when we see her at the end of episode 1. She seems to be about the same age as she is in the part of her tape where she boards the Train, so we can assume it hasn't been very long (?). There's no way of knowing how she got there, but in any case, she's no longer wearing the standard passenger uniform at that point.
Several days (at least) later, we hear her explain to One that returning all the passengers' belongings is a good thing, because it will allow them to individualize themselves. She clearly has a problem with the way the Train operates and how it treats passengers, but for now, she is still trying to influence One to see things her way rather than antagonizing him. Perhaps they were already working together like that before episode 1?
We can't completely trust what Owen Dennis says because he loves to shitpost during Q&As, and also because he strongly believes in the concept of death of the author, but I'm still sharing something he said on the subject that seems sincere.
Next, I'd like to talk about something that I never see mentioned as potentially related to the Train itself, and that's the green substance in that pipe in The Astro Queue Car. Obviously, given the rather ridiculous depth at which it is located, this is not something that is meant to be found by passengers (also I find it hard to believe that Ryan dug all that out with his bare hands, but whatever lol).
At one point, I thought that one clue that it was part of how the Train worked (and that it would be relevant later on) was that it was the same color as the orbs. When you put the two side by side, however, it turns out that the colors are different. So... maybe it's something else. Who knows?
There is a very thin theme that also links Amelia and Book 4, and that's music. We already know from contextual clues in Book 1 that Amelia controls the Steward using specific tones (also used by One-one on several occasions), and that this is a variation of the phreaking she uses to connect the payphone call without paying in the flashback in her tape.
That said, I can't help but notice that Amelia's takeover took place relatively soon after all the passengers retrieved their belongings. And those belongings include Min-Gi's stylophone. I wonder if the two events are related; for example, could she have noticed a glitch on a screen when Min was using his synthesizer and deduced that it was possible to phreak the Train's systems like phones? For now, it remains a headcanon, but I find the idea very appealing.
One of the most obvious patterns throughout the series is that something that will reappear in a Book (Lake, the Apex, Amelia) is always introduced or reintroduced in episode 7 of the previous Book. And, of course, the Docent is introduced in episode 7 of Book 4 - perhaps it (or maybe one of its victims) would have been important in Book 5?
(Incidentally, the headphones found by Kez clearly have no business being in that car and probably belonged to one of the Docent's victims, and a recent one at that, considering everyone just got their belongings back…)
Usually, when something isn't important, Owen Dennis shitposts about it, but his responses about the Docent are quite sincere and cautious. I don't know if that means anything, but it's worth considering.
Obviously we see when Amelia takes over the Train, how everything glitches when she controls every Steward at the same time, how she fumbles with the controls-
-and how she seemingly destroys most Stewards. Maybe it's all the Stewards that weren't the one that One-one was using in the Engine? We don't know. In any case, whatever she's doing resets Min and Ryan's numbers to 202 yet again. Did the same thing happen to every other passenger? Maybe including her?
Finally, we don't know exactly how Alrick died (aside from Owen Dennis's shitposts about a dirt bike accident, which I give about as much credence to as anything he says about non-canon character deaths lol. bless him), but we can assume that a hospital was involved, because we see bits of hospital rooms in several places in the background of Amelia's failed Cars. And Jeremy's backstory also involved an accident. Now I'm not saying that the two are directly related, mainly because I would be very surprised if Alrick had been dead for five years at this point, but there is an interesting parallel there as well... especially considering the fact that Jeremy also refused to move on and is also stuck in one place on the Train.
There's a lot more things we know about Book 5 from the various Q&As, but I was trying to focus on parallels we can theorize based on Book 4 specifically!