Bidirectional track circuit signal built on diagonal track doesn't work properly in half case. (´・ω・｀)
I made a small saved game (https://drive.google.com/open?id=1bbujwsb920vdQQKHtuUsk2IoRWciUr-4) for testing, please check it out.
On the west side there is a track on which signals are build on a diagonal track and this is Line #1.
On the east side there is a track on which all signals are build on straight track and this is Line #2.
I believe all signals on Line #2 are working properly.
However, you can see that the signal of Line #1 works differently than Line #2.
(That saved game is just before the train collides. If you want to check the signal behavior of Line #1, please the locomotive to the depot and run only one locomotive.)
The issue of Line #1 signal is:
(1) Bidirectional signal does not make any directional reservation.
(2) Although all signals in Line #1 are two aspects, some signals reserve more than two blocks like three aspect or four aspects.
As you can see in the saved game, the train may collide in a single track section of Line #1.
Collision is likely to occur in the south of the signalbox. I think that it could not illigally reserve a blank section on the north side.
Although the reservation indication is unnatural in the most part of Line #1, the place where the collision occurs may be quite limited.
Note that there are two positions to place the diagonal signal, but I intentionally built it at where the signal behavior broken. If the signal is built on the other position, it works properly. The difference in position becomes clear by displaying the grid. Rotating the map does not change the behavior of the signal.
EDIT: Sorry, I forgot to add the saved game link.
Thank you for your report. I have now pushed what I believe to be a fix for this. The problem was in building the signals, so the fix applies only to newly built signals. I should be grateful if you could re-test with the next nightly build.
I am sorry to reply late.
I confirmed that bi-directional signal built on diagonal track makes directional reservation properly in currently build.
Thank you for fixing. (´・ω・｀)b