I was in the process of upgrading some things.

New train, hauling sulfur from the refinery to the rocket production plant. Rockets are something that are only consumed when you're fighting so the plant is prone to filling it's buffers. I come back from expanding my borders and find everything deadlocked......there's the sulfur train sitting at the rocket plant. It's full of sulfur so the exit conditions at the refinery are met. It just sat there. The next train was trying to get into the rocket plant, it was backed up onto the main line.

Proposed solution: a train should not freeze because every station is skipped.