Best way to get a job with the railroad?

I'm currently 18 years old and I'm looking to work for the railroad. Any railroad would be fine, but ideally Union Pacific would be the best. I have wanted to drive a freight train ever since I was a little kid. When I looked at Union Pacific Railroad's website, I didn't see any opportunities. What is the best way to try and get a conductor job with Union Pacific or any other railroad? I realize that I'm not old enough to drive a freight train yet but I hope to get to that point in three years.