Mister Rogers' Neighborhood - Season 3 Episode 36
Death of a Goldfish

A fish in Rogers tank has died. Rogers buries it and, with Bob Trows help, constructs a tombstone with a pentagon that bears a fish drawing. In the Neighborhood of Make-Believe, concern hits when the Trolley stops at the Castle and doesnt move. Eventually, with help from Bill Platypus, they find out that the Trolley simply suffered a derailment, preventing full electrical pickup (as explained earlier by Mister Rogers).