Originally Posted by Darth Mario
Found your problem. Soul sand needs to be on the block just outside of the doorway, not the one just inside of it. Least, that's how mine work.
Well, I tried that. And it stops me being able to walk /into/ the minecart tunnel...but not /out/ of it, which is what I'm trying to do, to stop mobs wandering up the tunnel and into my basement.
I did manage to get it to work though, by building an arch on the basement side of the soulsand, then establishing the "door" on the other side. A kludge, but it keeps the zombies out.