If you have a postwar F3 with a bad horn, you can pay $35 for a Lionel horn, or buy a $6 bike horn and do a little work.
Three guesses what I did, and the first two don't count...
The factory horn dewired and adjustment screw removed.
Bracket clipped off and ready to drill!
Replacement horn fitted to the Lionel bracket.
A bit of soldering and heatshrink tubing later, the new horn is cheerfully beeping in its new home!
A search on eBay for "1.5v bike horn" will turn up several similar items, if you have a post war diesel in need of a new horn and you're impecunious like me...
Hope this helps! 
Mitch
