Depends. Somewhere, someone has specific language to determine. They may make it vague for you and have a pretty clear definition at Toyota proper.
In general, however, "powertrain" refers to anything along the power from the engine to the wheels. My definition of this has always been anything involved in the actual job of taking power from the engine to the actual wheel. Engine, tranny, driveline, diff's, axle, hubs, CV Joints and shafts, transfer case.
Steering is not, in my opinion, considered "powertrain". It steers the car, doesn't move it.
However... that doesn't mean Toyota doesn't include it. You need to get a hold of specific warranty language. Call Toyota proper..
