Two Yankees fans are on a train up to Boston to watch their team play the Red Sox. They start making fun of a couple of Red Sox supporters who only have one ticket between the two of them.
Just before the conductor appears both Red Sox fans go into the bathroom and lock the door behind them. When the conductor knocks on the door they slip the ticket under the door, the conductor clips it and slides it back under the door and off he goes.
On the return journey the Yankees fans decide to pull the same trick and purchase only one ticket for the two of them. They notice that yet again the two Red Sox supporters only have one ticket between them. The Yankees fans realize there is only one bathroom per carriage and quickly take the lead, locking themselves in first, leaving the Red Sox fans with nowhere to go.
A minute later the Red Sox fan without a ticket strolls over to the bathroom and knocks on the door.