If I tell you how to hack the DC transit system right here in this post, will DC issue an injunction to have slashdot remove the post? Let's find out!
In the DC system, you have to scan your card to get into and out of every station. Rather than having standard boarding fares like NY, it actually takes into account where you scanned in and where you scanned out and then deducts the appropriate amount for the fare between those two points at the time you scan out.
But say you leave the same station you entered. Maybe you missed your train and decided to take a cab, or forgot something, or got a call and changed your plans, or just want to rip off the DC transit system. Whatever. You always have to scan a card to get out, and if you scan the same card, it doesn't let you out for free, but charges you a minor fee. I think it was $0.25.
So, say you have a standard commute to work and back every day on the DC transit system:
Go into your point of departure and buy two cards, one with the appropriate fare to your destination. Swipe both of them in.
Ride to your point of departure. Swipe the exact fare card out and throw it away.
Go about your business at your destination. When you return:
Buy a new card and swipe it in.
Ride to your point of origin and Swipe OUT the card you only swiped IN at the same point earlier. You just rode there for $0.25.
The next day, swipe that same card in at the same station. Ride to your point of departure, and swipe out with the card you bought at that point yesterday. Another $0.25 trip.
Always continue to scan in and out at the same station using the same card. Every trip between those stations will be $0.25.
There is no expiration on how much time may pass between swiping in and out of the same station for the minimum fee. There is nothing set up to catch that one card is swiped in and out of the same station every day about 9 hours apart, while another card is swept in and out of another station about 15 hours apart. At least, not unless they've fixed it in the past few years.
Obviously, buy the cards you use for this with cash, not a credit card.
If you really want to be a cheap skate, quadruple your money
[schneier.com] also. Then all repeat rides in the system will be priced at approximately $0.07 each.