I copied the original post here so that I can read it:
"I bought a Lionel Type LW transformer and when I depress the Whistle button I can see a change in voltage on my layout but the two trains I tested do not whistle with this transformer. I tried an old #2055 steam engine and a new Strasburg #475 steam engine. The Lionel 2055 is typically hooked up to a Type CW transformer and since both the engine and the Type LW transformer are postwar I expected that they should be comaptible.
"I wired the transformer per the transformer's manual but did also test reversing the leads. When I open the transformer and depresss the Whistle button I can see that it slides down to make a new contact with a copper head. I do not see any loose or broken wires. I did have to reset the lever for the Direction button because it was working in reverse (making contact instead of breaking contact). So I am sure someone was under the cover before me.
"Does anyone have any ideas on troubleshooting this problem? It is a great little transformer but without the whistle button functioning I hesitate to use it."
As for the zener-diode suggestion, a zener diode is not an appropriate replacement for the original copper-oxide rectifier, unless it has such a high zener voltage that it functions as an ordinary diode at toy-train-transformer voltages. A simple silicon rectifier diode is probably the best choice. Its forward voltage drop is somewhat greater than the copper-oxide rectifier's, but not enough to make much difference. It is important to get the polarity right. The cathode should point toward the center rail, although Lionel always showed it the other way in their schematics.
The reference to a zener diode may arise from confusion with transient-voltage suppressors (TVSs), which are a good precaution for use with modern electronics-rich trains. However, a TVS is wired in parallel with the transformer output, not in series with it. It can be mounted inside the transformer, inside the locomotive, anywhere in-between, or in multiple places.