If you're tight for space on the subway and don't mind looking like a doofus on the street, Lutz's bicycle-cart saves you from owning a bike and a luggage roller. Bicycle by day, cart by later that day, the Carry-Bike is useful in both modes.
The next time some BART (that's the mass transit system here in the Bay Area) conductor gives you a hard time about your bike, change it into a carry cart and tell him that his job will never allow him to afford this fancy contraption. Then watch as he beats you with said cart.
Now available for $380 USD. We're just kidding about that conductor jab, too. Seriously.