driving wheel

from The Century Dictionary.

• noun In machinery, a main wheel that communicates motion to another or to others.
• noun In railroading, one of the large wheels (commonly four, though occasionally as many as ten, in number) in a locomotive engine which are fixed upon the crank-axles or main shafts. Also called driver and drive-wheel.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun On a steam locomotive, a powered wheel driven by the locomotive's pistons or turbine.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a wheel that drives a motor vehicle (transforms torque into a tractive force)