A guy named Pete gets a job as a switchman with the railroad, and undergoes weeks of training.  The supervisor then takes him into the switch tower to test his readiness.  The following exchange takes place:

Supervisor: "Imagine you were sitting here alone and you learned there was a train coming from the North on that track, and another coming from the South on the same track.  What would you do?"

Pete: "I'd throw this switch right here and put one train on the other track."

Supervisor: And what if that switch didn't work?"

Pete: "I'd go down to the track and throw that big switchlever there, putting one train on the other track."

Supervisor: "And what if that switchlever didn't work?"

Pete: "Then I'd come back here and call the dispatcher to stop both trains."

Supervisor: "And what if the phone didn't work?"

Pete: "Then I'd go to that gas station across the street and use their phone."

Supervisor: "And what if their phone didn't work?"

Pete: "Then I'd go get Uncle Joe."

Supervisor: "Uncle Joe???  What would he do?"

Pete: "Nothing, but he ain't never seen a train wreck."