Maybe there'd be a serious road system between, say, 3 and the Capitol -- technology seems like something the Capitol would want constant access to. We never hear Katniss talking about any roads into the Capitol from the attack route in 13. If the Capitol is on the Western slope of the Rockies, that would also be a limiting factor -- roads through the mountain passes are notoriously bad for upkeep. So maybe, from the east, there's just the one train.
But you bring up an interesting point about the tracks that small numbers could move on. I'd bet there are quite a few around districts like 2 or 10 or 7, where there's a lot of internal ground to cover.
Towns and cities are pretty arbitrary here, often defined by the state. Technically, my village was mostly in a town of hte same name, though part of the village was in the next town, and also in the town were several hamlets -- mostly crossroads where a few people had built houses. Then there was the "city" just north of us (Batavia) where the "Entering the city of Batavia" sign was right by the silo of a barn on a big dairy farm.