And I still can't fathom what the hell a "steerable" locomotive truck does... Is this for those rare occasions when the do a little "off-roadin'?"
A "steerable truck" is really a misnomer (like "shock absorbers," which don't absorb shocks); it should really be called a "self-aligning axle truck," but that's a mouthful. Trains run most efficiently when the wheels are exactly perpendicular to the rails. When the rails curve, wheels in a rigid truck frame are no longer perpendicular to the rails. A "steerable" truck alters the angle of each axle to maintain perpendicularity; the amount is quite small given the large radii of curved track, but a little bit is all it takes. This improves efficiency and reduces wear and tear on the wheels and rails.
I couldn't find an illustration online quickly, so I tossed this together. On the left is a conventional truck; on the right is a steerable truck. Curves and angles are exaggerated for effect.
