MDs and PhDs

4 MDs and 4 PhDs are going to a meeting by train. The 4 MDs each have their own ticket, but the 4 PhDs (who have little money, of course) have 1 ticket among them.
The MDs ask the PhDs (in a caring manner), "How are you going to manage with just one ticket between the ten of you? "Just watch," reply the PhDs.
They all get on the train and the 4 MDs take their seats and hand their tickets to the conductor. But the PhDs all pile into a bathroom, and when the conductor comes by, a single arm reaches out and gives him the ticket.
The MDs, feeling enlightened, decide to try the same thing on the way home, so they purchase just one ticket among ten of them. The PhDs buy no ticket at all.
"How are you going to get home?" ask the MDs.
"Just watch." the PhDs reply.
When they get on the train, all the MDs pile into a bathroom. 3 of PhDs get into another bathroom. The fourth Ph.D. then knocks on the MDs' bathroom door and says "Ticket please."