Because it's one of those classic "The story is about what happens when we accept the conceit of the setting" sci-fi approaches. It's a train, because there's exploration to be done around what happens on a journey that is both linear and fixed.

> It's a train, because there's exploration to be done around what happens on a journey that is both linear and fixed.

Eh, they could have done that with a shelter built in a tunnel or something like that. But that would have been boring; it's a train because trains are cool. And they could have had the train circling a city or even continent, but they made it circumnavigate the world instead because that's cooler.

It's based on a French visual novel and the theme is class struggle. Trains are a pretty good metaphor for that as trains segregate passengers by their "class", usually defined by how much they can pay for the fare.

I haven’t watched the Netflix adaption but I think they were trying to suggest it’s a fixed loop with no driver. That felt thematically appropriate

is it the dumbest concept in all fiction?

No. That would be machines have taken over the world but thier only source of electicity is farms of dreaming humans.

Hilarious and appropriate but also poetic

The dumber part of that world is that there was a separate train of very similar or identical design that managed to defeat the "can't stop for prolonged period of time" plot point using nothing but what was available on the train, and it was somehow able to do this only after the world ended and not before.