Lionel engine shell wire location

Hi there all. Been reading for months but this is my first post. Getting back into the hobby after many, many years.
Was inspecting a used loco I purchased and a wire taped to the inside of the shell came loose from the board. The question is which terminal does it go back onto?
My second question is the purpose of it? I don't see where this loco has sounds if it acted like an antenna and the shell is plastic so it doesn't ground anything?  Please enlighten me.

Thanks,
Rob