Build something like in the attached screenshots. When sending a minecart from the Nether, it moves along the U-turn but refuses to travel back a second time, i.e. it simply gets stuck inside the portal block.

You could also build a railway track to a completely different portal. The behavior is the same there. (I tested that as well.)

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Screenshot overview:

• 2013-03-21_23.51.34.png: The basic setup. Send a minecart from the nether to this portal.
• 2013-03-21_23.52.13.png: A minecart that got stuck in a portal block after moving along the U-turn.