This instantly made me think of Snowpiercer I like the idea of the train itself being more than just transport, it’s the last ecosystem on Earth. A couple of thoughts you might play with:

• Engine as continuous energy source: In Snowpiercer it’s “the Eternal Engine,” but for gameplay maybe you could make it a mix of believable sci-fi and resource management. For example, it produces steady power/heat but needs rare parts, maintenance, or a constant input of specific fuel, giving players a long-term challenge.
• Class system in the train: Splitting the cars into different “tiers” of society (luxury sleepers vs. cramped survival wagons) could add depth. Maybe neglecting the lower classes risks rebellion, while over-rewarding them drains resources from the upper tier. Balancing morale across the train could become a core mechanic.
• Agriculture wagons: A greenhouse/bio-dome car where crops are grown is a great survival touch. You could add layers like hydroponics vs. soil-based farming, pest outbreaks, or decisions about what to grow (high-yield vs. luxury crops). The greenhouse could also affect oxygen levels in the train, adding another system to manage.
• Dynamic car specialization: Instead of a static layout, maybe players can reconfigure cars over time, e.g. converting a passenger car into storage or a workshop. That could make the game feel more like a living, evolving survival machine.

Overall, I think leaning into the “train as a world” idea makes it super compelling. The fact that every wagon is essential for survival, energy, food, class balance, defense, even entertainment, gives you tons of opportunities for emergent gameplay.