It is known as a 4-8-8-4. So the first truck is a guide for the engine....next comes the first engine, (the 8 part) then the second engine, then the last set of wheels...(the 4 part) they are there to support the weight of the firebox. So while the entire engine is rigid, the front engine swivels on a hinge located just between the two engines in order to accommodate curves....the front bogie truck as a part of the lead engine assembly......here is a video of them, the part you are referring to starts at .055 into the video...enjoy....
http://www.youtube.com/watch?v=x8f9VFlNyDQ