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:

|
WD
ABC

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.