I have postponed this for quite some time, but it's time to adjust some of my cargo routes so they will become more efficient.
One thing that needs to be done is splitting all refrigerator and non-refrigerator goods over two different trains for each city.
I hope by doing that, that a city will be better supplied with all goods, which in turn will make it easier for them to grow.
And although I wasn't able to see all of its effect, it looked as if things were working well indeed. Except I forgot to apply this to all cities in my network, which means I will have to the rest again in the previous episode.