Upgrades attract traffic, it seems, and some parts of the rail system in the cluster are bursting at the seams.
Here is an example, found in Arcens, the center city in the cluster. The rail station in the next picture got upgraded very recently. The statistics after a few months of gameplay are quite worrying:
The pair of dual tracks going to the top of the picture link to the neighbor city of Deramey - quite exactly to the rail triangle that was recently upgraded in my recent CJ entries. The volume statistics on those pairs of tracks are also of great concern:
On the left pair of tracks the volume is - unsurprisingly - 65535. This means that the link is saturated, which in turn explains the excessive usage rate of the train station.
Aside from upgrading the station again, I could put in a third pair of tracks, but I find this rather unrealistic. Another option is to add one or more subway links. I am undecided at this point.
What this tells me is that I am encountering more and more saturation issues in inter-city traffic, and it may put an premature end to my work with the cluster.