[brian51]

Of course I am not the wiser, as to why the abandonment happened, and why it ceased. For the moment I'll consider it as a fluke in the game play. If it occurs again I'll try to look into things in more detail.

I always felt there are also cyclical recessions that vary in strength in the game
So maybe this is at least part of the reason ?

I never expect that numbers will jive exactly, since cities can't be played simultaneously - there is always a delay effect. But in my previous observations the differences between both sides of an inter-city subway link were rather minor, on the order of a few percentage points. Here we have major discrepancies.

Well, I must live with that, since I intend to forge ahead with the development of the cluster.

Yes, we have always known here are some traffic bugs in the city connectors.
Either this is in the traffic reporting or the actual traffic, or both .

I do have one suggestion though that may or may not help.
I have heard it makes things more difficult and less accurate if you have crossing networks that are near the edge of the map , where the city connectors are. So in marlande, you have that one crossing subway tube fairly close to the edge of the city border there. If there was some way you could move that a  few more tiles further away from the border, I would be curious if this helps balance the traffic more ?