Quote Originally Posted by The Bushranger View Post

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.
Sorry, misunderstood the problem. The solution for that would be to build the one-way door further down the tunnel, just before the unlit area. You can also put half-slabs at face-height in the tunnel, above the tracks. Slabs won't suffocate you and they'll allow you to pass through in a minecart, but not when you or the zombies aren't in one. Glass, glowstone, or stairs would work as well, though those would have the side effect of allowing you to see through walls as if you had x-ray.