In Beta 1.5 the build limit is 127, so you can't place anything on the nether roof. You can stack 3 minecarts though, with the correct setup they propel each other. You have to make sure that they are on a rail, otherwise they will be on top of each other instead of inside each other. Also, you can't place a rail, and just place multiple minecarts on it like in modern versions; here the minecart hitbox completely covers the rail

You have to be careful though, because any entities that come in collision with the minecarts (items, mobs, etc.) will unstack the minecarts, and you'll have to start over. (You can see the dropped items changing direction of the minecarts)

In a run, I dug out a 1000 block long tunnel, so the minecarts are safe. It's probably faster to make a bridge with railings and small tunnels above the ground.