I use two bikes. A 16 inch wheel Dahon Piccolo for 6 blocks to the train. This bike is great for short commutes since there is nothing on the bike worth stealing unless you have another 16 inch Dahon.

When I get off the train, I ride a cheap toy store bike (Paicfic MTB) from Toys R US. I ride this heavy junk bike for 1.2 miles to work. The MTB has lived on the streets of Manhattan 24/7 now for six months and is practically theft proof in the stolen bike capital of the world