Why aren't my residential zones occupied?

I started building this city in the top-left corner of the map, and expanded first down, and then to the right. Recently, I noticed that some of my residential zones (eg. the cluster in the bottom-right corner) haven't been built on, while others (eg. the group visible in the main window) used to be occupied, but aren't anymore.

What's going on here? It isn't a lack of demand (the CRI indicator is showing maximum demand for all zone types). It isn't a lack of access to other zones: some of the unoccupied zones are adjacent to both commercial and industrial. I don't think it's my use of railroad in place of road, because I didn't start having trouble until recently.