Let's name the movable rail that can be pushed by piston A, a second swithing rail B, a stationary rail C, and the "source" rail, where the Minecart should come from D, placed like this:
|WDABC
Where | is the piston, aiming the wall W.
An activated redstone torch is under B to link the rail D to C. If the torch is off, the rail B links the rail D to A. This works. Now, if the piston | pushes the wall W which pushes the rail A (when the torch under B is off) the rail B is still linking D to the wall.
What I was expecting is that when I turn the torch on, the rail B links rail C again. Instead, whatever I do, it's still linking the rail D to the wall.
This can be fixed by recomputing the rail's shape when the neighbor block changes (I tested).
Thanks, Nicolas.
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In this world I simplified the system to make it easyier to understand, but on my survival vanilla server, the piston moves the rail to hide it, and make a "secret passage"; when the "secret passage" is closed, the minecart goes from rail D to C, else from the rail D to A.
I know that the rails must be in a T shape to make the switch working, but even if I push the rail in the original position, and all the 4 rails A, B, C and D are making a T shape again, the switch rail B does not move. I'm sorry, it's hard to explain: I'm french, and here everything is in english.
Can't reproduce. If I push the rail back and it forms a T shape again I can switch the junction (with a redstone signal).
What you have is a timing issue. Switch the junction first (with a redstone signal). Then push the rail away. Then do the same the other way around.
Everything works as intended. Re-computing the rails with every block update will break a lot of constructions. No bug here.
What's the reason for having a piston there? Why not simply power the rail at B with redstone?
What you show in the last picture is intended. It's not possible to switch a curved rail if it does not have three other rails connected. It must be a T shape to work.
Summary: Everything works as intended.