Thanks, both. This explains it. I was thrown because "between 0 and 0" makes no sense. Now I know it just means "any length", I can go ahead and add length limits to the stations. And train limits sound like an excellent idea too. That will definitely improve things.

I guess it must be a mod ...

This message was getting spammed at me from different stations. I have no idea how it started, and I ignored it at first. Then it became just too frequent.

Eventually, I discovered a busy station which had no space for trains to queue. The backup blocked a main route. The station also had ...