Focusing on the theme can help immensely. You described it as:
(The main point of the story is how each person sees the world differently and interprets the objective reality through subjective lenses)
All prior stories I am familiar with that explore this theme use overlapping timelines, e.g. the same event told from 4 different characters' POVs. You have the impressive goal of conveying this theme with no overlapping timelines (if I understood your description accurately). So, in addition to character jumping, it would help to develop the theme through other means. This includes asking tough questions like: why a train crash? Why not some other major event? Does a train have associations that support your theme? Maybe not, but those sorts of questions can lead to other things that do flesh out the theme. A couple possible images that point at subjectivity, as examples: fractured windows (not a mirror, that usually lends itself to a theme about Identity/Self); the "telephone" game - as in, perhaps in the crash aftermath a series of miscommunications occur due to characters' subjectivity.
Not coincidentally, this should also lead to further plot and character development.
From the limited description, I would suggest that the crash is not the climax; instead, it would be the first turning point in the plot. There could still be a series of increasingly dramatic turns, before the climax. The climactic scene would reveal the theme's effects, which means that you may need to expand the theme one step further: is the main point that subjectivity merely exists, or that it is good, bad, necessary, humorous, etc? I have a suggestion that I'll describe after one more point.
I disagree that the teenager's character arc would require her to mellow and help the other passengers; it is entirely plausible that the character's reaction to a major traumatic event could be apathy, or even a further inclination toward harming others. As you think through the thematic elements and use that as a creative source for plot and character development, it's also crucial to fully "know" your characters separately. I'm guessing you've done this, but I find it helps to write out much more of their backstory than will ever find it's way into the story. Understand their personality type. Understand the way they think and process things, from simple mundane activities (what type of driver are they?), to how they'd react in a train crash. When pondering a character's personality, I like to purposely create a contradiction, for example an extrovert who loves to code software. 96
Then I think of how that contradiction could resolve (maybe they bring their laptop to bars and code while chatting with strangers). Every real person I know has contradictions like this. You seem reticent to let the characters have arcs, but perhaps instead you just need them to have the "right" arcs at the right times. ("right" to you). Understanding them more deeply will help.
So, one suggestion: perhaps the predatory teenager with "social anomie" learns that the crash was caused by a suicide bomber, and she becomes intrigued enough to join a terrorist cell. Unlike her fellow terrorists, she is there not because of an intensely passionate, deeply held belief system; instead, she is attracted to the idea of causing destruction to others and herself in a dramatic fashion. Meanwhile, perhaps the "adult conspirator" you mentioned could be the teenager's aunt or similar mother-like role. This character might at first be cautiously supportive thinking it was some sort of long con, but then becomes justifiably concerned. The climax could be the teenager successfully bombing a major population center like a mall or something.
Thematically, when describing the climax, the teenager's experience leading up to the event is euphoric, seeing the objective event in a positive way. Immediately afterward, the aunt character could experience the event in a highly negative way. By so doing, you wouldn't violate your non-overlapping timelines rule, but would still illustrate the same event in two diametrically opposed subjective POVs.
This is just one suggestion, but hopefully it illustrates a method of using your theme to weave together a large number of characters' perspectives in a cohesive manner without trying to resolve every unique premise.
One last note: you mentioned two things that compete in my mind: on the one hand you seem to have set up a story rule for yourself that every scene is told from a new character's POV. On the other hand, you would like to fully develop the jihadist and teenager POVs. I ran with the latter concept (and chose the teenager since the jihadist dies). Using my suggested plotline as the example, one possible resolution would be to tell the teenager-joins-the-terrorists story via all the surrounding characters: a friend, then one of the terrorists, then a mall-shopper, etc. That way, you focus on the teenager story without violating the "every scene shown through a new character" rule. 11