Piston Minecarts are mechanical minecart with a piston in it. It is like the powered minecart or storage minecart, but with a piston. When it goes over a powered activator rail, the piston extends. Just like a normal piston, it pushes blocks. Sticky piston minecarts also exist.
You craft it like so (P = piston, SP - sticky piston, MC = minecart, 0 = nothing):
A Piston Minecart can be used when you need to activate two pistons, one after the other, with only one piston.