I hopped in a minecraft and rode a track, that I made some time ago. When I arrived at the end of the track I wasn't able to leave the minecart or to hit it with a sword/bow or other items. I tried to ride back and to place water before me without success.. I had to place Lava to leave the cart but that killed me then.
Additional Observation: When I walked back the track to get the rails I broke with the water, they were gone. I'm 99% sure, that the despawn time wasn't over, though.
Steps to Reproduce:
1. Enter a minecart on powered rails.
2. Push a button for a trackselection; this button is not connected to the powered rail, but to a railway switch later on the track
3. Push the button to start the ride.
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Minecarts, boats, and pigs are now dismounted by sneaking, same as horses. A message informing you of this fact pops up when you mount something.
Came here to report the same thing. I was in the nether, and had to drill down to lava to die. I could break all blocks around the mine cart, but not the cart itself.