Automated passive mob collection system for 1.8

My base is surrounded by water and I wanted to collect some passive mobs in preparation for 1.9 breeding. I decided to use the fact that passive mobs spawned in new chunks will no longer despawn. I figured I could automate the collection fairly easily allowing me to go off and explore/build/gather resources etc. A railway does this very effectively.
You would set up a system in the nearest chunks containing passive mobs with a rail going back to base. You could then just leave it running until all the available mobs have been captured then rip up the system and move it further out to the next area extending the return rail. Kind of a painful process but far less painful than making a MASSIVE spawning tower that only produces a new mob a few times a day.