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TL;DR Train has reserved a path through a block and the signals are yellow. The train then runs out of fuel and comes to a stop before entering the block, but the path remains reserved and is preventing other trains from entering the block.

Reproduction
1) Load this save file (110MB, self hosted)
2) Go to -268, -166, Nauvis (linked in chat, also in image below)
3) Train is not in the block, has no fuel, is not expecting fuel, and has 0 speed; but still has path reserved.

The Issue
A block is not expected to be reserved if a train has stopped moving outside of the block, it should be unreserved once it becomes impossible for the train to enter the block without external intervention.

Thanks for the report however it is expected that trains will continue to have fuel to run. As such, they reserve blocks with the expectation they will keep moving and if that fails, then they may get stuck like this. You the player can easily resolve the issue by requesting more fuel into the trains through robots, or by putting fuel in yourself.

It's not worth the additional code complexity and edge cases that will arise to try to prevent this.

See also viewtopic.php?p=339902#p339902