Hi JNixon and welcome to The Gauge!
The Lionel horn in that engine isn't a complicated one. First I'd make sure that their isn't any corrosion around the battery contact points. If the contacts arn't really clean the horn won't work. While you have it apart look for a wire thats come unsoldered, pinched or broken.
Those are the only items that I'm aware of that would cause it not to work.
Let us know what you found out.
Rich
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Rush Run River Logging Co.
