What exactly is in the Lionel 10-5906-001 Sound Activation Switch? I find very little on it, but some obscure reference mentioned a D-Cell battery. I have a Rail-Sounds boxcar, and I'd like to get a reverse polarity DC bias to trigger the bell. If this is just a D-Cell with a swtich, I might consider building one, since I've been unable to locate one for sale. I'm guessing there might be a component or two to allow the A/C to pass while injecting the DC signal.