Rails: How do You Control Juntions?

Hello everyone, A few friends of mine and I have gotten to the point in our multiplayer world where we've got several important locations connected through tunnels in the Nether. We're kinda old-school so we use rails in spite of the presence of more efficient options... thing is none of us are experts on rails. Anyone know how to set up a T intersection so that incoming traffic from two directions is always sent to the third and from the third either of the others may be selected?