Three engineers and three accountants were traveling by train to a conference. At the station, the three accountants each bought tickets and watched as the three engineers bought only one ticket.

"How are three people going to travel on only one ticket?" asked an accountant.

"Watch and you'll see," answered an engineer.

They all boarded the train. The accountants took their respective seats, but the three engineers all crammed into a restroom and closed the door behind them. Shortly after the train departed, the conductor came around collecting tickets. He knocked on the restroom door and said, "Ticket, please." The door opened just a crack and a single arm emerged with a ticket in hand. The conductor took it and moved on.

The accountants saw this and agreed it was a quite clever idea. So, after the conference, the accountants decided to copy the engineers on the return trip and save some money (being clever with money, and all that). When they got to the station, they bought a single ticket for the return trip. To their astonishment, the engineers didn't buy a ticket at all.
"How are you going to ride without a ticket?" asked one perplexed accountant.

"Watch and you'll see," answered an engineer.

When they boarded the train, the three accountants crammed into a restroom and the three engineers crammed into another one nearby. The train departed. Shortly afterward, one of the engineers left his restroom and walked over to the restroom where the accountants were hiding. He knocked on the door and said, "Ticket, please."