Three lawyers and three engineers are traveling by train to a
conference. At the station, the three lawyers each buy tickets
and watch as the three engineers buy only a single ticket.

"How are three people going to travel on only one ticket?" asked
one of the three lawyers.

"Watch and you'll see," answers one of the engineers.

They all board the train.  The lawyers take their respective seats
but all three engineers cram into a restroom and close the door
behind them. Shortly after the train as departed, the conductor
comes around collecting tickets.
He knocks on the restroom door and says, "Ticket, please." The door
opens just a crack and a single arm emerges with a ticket in hand.
The conductor takes it and moves on.

The lawyers saw this and agreed it was quite a clever idea.  So
after the conference, the lawyers decide to copy the engineers on
the return trip and save some money.  When they get to the station,
they buy a single ticket for the return trip.  To their astonishment,
the engineers don't buy a ticket at all.

"How are you going to travel without a ticket," asks one perplexed
lawyer.

"Watch and you'll see," says one of the engineers.

When they board the train the three lawyers cram into a restroom and
the three engineers cram into another one nearby.  The train departs.
Shortly afterward, one of the engineers leaves his restroom and walks
over to the restroom where the lawyers are hiding.  He knocks on the
door and says, "Ticket, please."