The Lone Ranger (William Conrad) and Tonto (Ivan Naranjo) escort a prototype refrigerated train headed for Denver, but two outlaws, one of whom was already captured by the Ranger, decide they want the train for themselves. "The Runaway" was the 1st season finale, broadcast in January 1981. While not credited, Frank Welker, Alan Oppenheimer, and Lou Scheimer are also heard.
You might say Filmation was ahead of the PC movement by having Tonto speak perfect English in this series. I think Jay Silverheels, the definitive Tonto in many fans' minds, would've approved.