An articulated steam engine is essentially two steam engines in one frame, and it can "hinge" in the middle. The arrangement gives lots more power, but requires the hinging to get around curves.  You'll know it's an articulated by the sheer number of wheels.  They're usually designated as 2-6-6-2's or 2-6-6-8's, but always four numbers, as opposed to usual steam engines that just have three digits (ie, 4-8-4) in their classification.  There's a Wikipedia page on articulated's here, but we found it kind of arcane.  For more on the locomotive numbering scheme, ie. 2-6-6-2, a.k.a. Whyte Notation, see this page, also at Wikipedia.