I just digged through some older postings and found an old one from me which pointed out some problems with bidirectional stations: http://www.tt-forums.net/viewtopic.php?p=278388.
Now I have thought about that problem again, and I think I came up with a solution which is more general.
Basically, I want to set up a station like what's shown in the attached picture. Trains should be able to use both platforms in both directions. Now, there is no signal setup which avoids that trains try to get to the same platform from both directions. To do that, the signals at either end of the platform need to be connected somehow, i.e. signal a and c as well as b and d need to depend on each other in some way.
Some theory: The two-way signals are necessary as the station tracks need to be accessible from both directions. Trains only reserve a path towards a green two-way signal, i.e. when a path towards a station platform is reserved, the platform is free. Still, trains can reserve paths towards the same platform from both directions.
Idea: If a path towards a two-way signal is reserved, the block behind it (in the picture this would be exactly the station platform) needs to be marked as blocked, as if the train approaching is already in it. This would trigger the two-way signal at the other end, so that no train from the other direction tries to get to that track, except it reserves the path at exactly the same moment, which I think is somewhat unlikely.
This should work regardless of what the next block looks like, if it's a station or not, so it might be just general enough to actually work. At least I did not find a track combination which would not work anymore, apart from setups that would not work anyway such as using two-way signals everywhere. But maybe I'm just blind, so feel free to point me to a setup that this suggestion would break.
Quite a long post, thanks for reading
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