"Where did you get that?" asked the first.
The second engineer replied "Well, I was walking along yesterday, minding my own business when a beautiful woman rode up on this motorcycle. She threw it to the ground, took off all her clothes and said 'Take what you want.'"
The second engineer nodded approvingly "Good choice, the clothes probably wouldn't have fit."
Then, one of the engineers said "here comes the conductor" and all of the engineers went into the washroom. The mathematicians were puzzled. The conductor came aboard, and said "tickets please". He checked all the mathematicians' tickets, then proceeded to the washroom, and knocked on the door. The engineers slipped the ticket under the door.
The conductor left and the engineers came out of the bathroom a few minutes later. The mathematicians felt really stupid. On the return trip, the group of mathematicians bought one ticket for the group. They snickered at the engineers, for the whole group had no tickets at all!
Then, the engineer lookout said "Conductor coming!". All the engineers went to one bathroom. All the mathematicians went to another bathroom. Just before the conductor arrived, one of the engineers crept over to the mathematicians' washroom ... knock knock.