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Every once in a while I will get a train which is deadlocked with itself on a roundabout. The train will try to make a 450 degree turn and thus run into it's own tail. I'm not entirely sure what is happening but this is my best guess.

I have deciders which disables a station when there's a train at that station, this will force a train to reroute. When this reroute happens while the train is on a roundabout the pathfinder could try to go into a direction which is blocked by the train itself.

/edit:

Do not have any mods installed (100% vanilla).

Most likely a duplicate of: viewtopic.php?f=41&t=22939

In case this issue gets some attention, here is a very minimalistic setup to reproduce it:
The train has been (manually) put into a position where it deadlocks itself when trying to reach the station. Let's put aside the can of worms which is the train freeing itself by ignoring the signals or changing the behavior of the signals. In this setup there is an alternative, longer path available which does not require passing through a block reserved by itself.

I'd consider the "ideal" solution to this issue being that trains reject a path which intersects a self-reserved block, if an alternative path is available it takes it otherwise it displays "No Path", that way trains and signals can continue to interact in a very absolute way.