Battery question ??

I know you can pick up power from the track. But I guess that you are getting around 17 volts DC. At least that is what I think. My train transformers put out variable DC output up to 22 volts. I want to eliminate using batteries (3 AAA = 4.5 volts) in the tender of my Keystone/Bachman engine that provides power to the chug-chug and whistle sounds. My question: Is there "something" that I can wire in that will reduce the track voltage to 4.5 volts to eliminate the batteries? I do not want to blow out the sound board by blasting it with higher voltage then I should. Has anyone done anything similar? The engine I want to modify is pictured in the video of the Christmas tree train I put on the blog last week.