Since we've been here, we've spent a fair amount of time on trains, subways, and trams. We were surprised to find that things work more or less on the honor system.
You can walk into the subway, go down the stairs, and hop on the train. No turnstile. No ticket taker. The trams have a driver and occasionally someone in the back who will sell you a ticket, but I've never seen them call anyone out. And on the commuter trains, I've only had a ticket taken on a longer trip. I've never seen a ticket collected between the airport and central station. The idea is that they take tickets only occasionally, and they enforce compliance by charging fines if they find you without a ticket.
There are other types of tickets, but for the most part we use something called a stripkaart. You buy your card at a newsstand. They come in various sizes, but typically have about 15 lines on them. When you get on a subway or tram, you're supposed to use a machine to stamp the date and time on one of the lines on your stripkaart. If someone comes around to make sure you paid, they'll look at your card to see that you stamped it. Once you run out of lines on the card, you need to get a new one. (it's actually a little more complicated than that, because depending on how far you're going, you may be required to stamp two to four lines just for one trip).
So we were on our way back from running errands and we went to take the subway. Our stripkaart only had one line left on it and we needed four. We couldn't find anywhere to buy a new one, so we just stamped the last line and got on the train. This was the first time in all of our riding that we had "jumped the turnstile". And wouldn't you know that about five minutes into our ride two policemen entered the front of our car and started checking tickets! I seriously almost told Beth to get off the train as quick as we could...if we hadn't been in seats, we might have. When they got to us, I showed him the stripkaart and was about to start making our excuses ("but we're Americans - we didn't know!") but he just said thanks and moved on.
The next day we bought the jumbo stripkaart -- we're set for a while.