In terms of steer-able trucks, from what I know, they basically allow larger 6 axle locomotives to operate on tight radius tracks. Having 3 axles per truck limits how tight the turn could be before the train either derails, damages something, etc. A 4 axle locomotive can negotiate a much tighter radius than a 6 axle, obviously. The steerable truck allows for the wheels to pivot in the truck so that they can move to accommodate a tighter radius like radius's that only 4 axle locomotives could operate on perviously, allowing the 6 axle locomotive to operate on a wide variety of radius's in a railroad's service area, saving money.
This is what I know about that type of truck, if someone wants to add or correct me, go right ahead!
