QG, The "electronics" of those old pw whistles was a relay which would pull in in the presence of direct current, or an unbalanced AC. A makeshift controller was to put a carbon-zinc D cell into the track circuit, which would mean half the sine wave was 3 volts higher than the other half, and this would trigger the relay. Only sine waves were available from the PW & pre-war transformers.
Thanks Ted. You are correct, they are different. The PW tenders do not use a circuit board.
