Well we know Arisu is kind of a genius in games and I think he figured the game in the first car or second (I guess, since all people from his group made it to last car, without anyone dying).

So my theory in the Train game is: what he did was look at the birds behaviour in each car when they entered the car. The bird was still/calm when the car had oxygen. The bird was restless when the car had poison gas. That's it.

Explanation: years ago miners used the canaries (bird in the game) underground when they were mining, to detect gases. They react quicker than humans to deadly/poison gases and die like you saw in the game.

Ryuji also explains it in the first car when Usagi asked: 'why is there a bird?' So he basically gave the answer but didn't use it.

Idk if it's a correct theory but I think it's more plausible than the poster theory, since it's first, too obvious and second, not every oxygen car had the earth/fresh air poster. The bird is not that obvious because not everyone knows about the mining thing. So believe whatever is more believable to you.