35v 1000µF cap across the DC outputs of the rectifier works like a charm! Amusing side effect: Since the relay is wired to feed AC to the rectifier, if the tender is placed on the track by its lonesome and the whistle is blown, the cap keeps the relay locked and the whistle keeps going. Doesn't happen with a loco attached, but I'm gonna have to revisit the wiring just a eentsy bit.
Mitch