I love the idea of the subway as a setting. My entry also uses a mechanic where the GM rolls for the number of "encounters" upfront, but the linearness of a subway system really lends itself to that idea. I like the idea of commuters as characters too. I think this probably cries out for either simpler (a route has 4d6 platform stops) or more complex (a subway route generation rulekit) rules governing the creation of the line.
I think this could be made into something really fun and unique, though the dead sun is implied at best. I hope you keep with it.