What is this type of crane, and what is it used for?

A few times a week, I drive past a construction site where there is a type of crane I’ve never seen before. It looks like your typical mobile construction crane–I think it’s mounted on tracks–but at the top of the boom there’s a hinged connection to another, equally long boom, which slants away from the crane back down toward the ground. The far end of this second boom rests on a pair of wheels.

Unfortunately, I can’t find a picture of one by googling. Anyone know what this is and what it’s for?

I’ve seen one of those, I remember thinking about how big it was since the base of the crane and the other end (where it came back down and touched the ground) where about a full city block apart.

My guess was just that it was to make one REALLY BIG crane by straightening it out and the wheels where just there so that it can be bent and flattened with the other end on the ground.

From your description, Luffing Jib comes to mind. Here is a link to one.