That's pretty much what it means, Aaron. As steam engines developed, they sort of ran into a roadblock: everyone wanted larger, more powerful engines to go faster, climb steeper grades, and pull more weight. The problem was, how do you make them larger? You can't really go taller or wider: the existing tunnels and bridges ruled both of those out. You could go longer: a longer frame allowed you to have a longer firebox and boiler and increase your power. Add extra wheels to support the weight as well as getting more iron against the rails for traction. The only drawback was that the curves in the rails limited how long you could get. When you have 4 drive wheels, you can cut a pretty tight curve. Those big puppies won't flex, though, so when you have eight drive wheels, you've cut your turning radius in half and can't go places you'd like to. The solution was to create a sort of hinge or pivot point under the engine to split the drivewheels into two groups, the front group being able to pivot. It required that they have their own sets of steam lines and valves, a pretty complicated arrangement. One of the early ones was designed by a Frenchman named Mallet (pronounced Mal lay). Sometimes you'll hear them called Mallets, like the hammer.
