Thanks for all the responses. To clarify a few things:
- The cities were not supplied by other cargo
- The frequency for the goods routes to the cities were all <2min (2-3 trucks)
- The train frequency was 6min, and I use a single big train.
After staring at my non-producing coal mine for a while I noticed that *south* of the coal mine there was another steel mill. So I simply set up a new rail down there. It ended up with the same frequency for the train (6 min), and lo and behold, production started.
I changed nothing else than the train line.
But, the southern steel mill was fairly close to a city. This city was the first to start pulling production from the steel mill. So it’s as if the setup I had was to “complicated” for the chain to get going, but once there was a nearby city production started up. And once production started the other cities started pulling as well.
To me this is a bug since the cargo delivery times were the same in both cases.