You know that time you had a really good idea....

https://images.steamusercontent.com/ugc/15511023551681996427/EFC56B5755EA2D55105589C0062CB30F3D559101/

I needed a train to loop around through two stations in my ingots complex to pick up from two stations. No problem - bung in a loop back to the inbound track. I decided I probably needed some extra signalling to prevent deadlocks.

I guess I didn't go far enough. Oh look - deadlock!

Moral of the story - if you have a visit from the train-based Good Idea fairy - make sure you add enough pathing signals to make it work.