AC versus DC Relay Question

There's nothing in a plain DC relay to prevent it from chattering on AC. 

The Lionel horn relay is a special relay with a copper ring to act as a shorted transformer secondary and swamp out the AC component to the relay will only react to the DC component of the signal.  The DC component in on the tracks is not affected by the shorted transformer turn and thus picks the horn relay.

I knew there was a good description of it somewhere, thank Rob.

Johnsgg1 posted:

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Any 'Chicago' Flyer collectors have a spare Prewar Flyer relay laying around?

Is this a restoration or a repair?  If it doesn't matter what's under-the-hood you can replace the unique and possibly hard-to-find DC-sensitive relay with a modern circuit module as described in this thread. 

In your S thread it appears you're still messing with the reversing-unit mechanism.  Seems one approach would be to install a switch to bypass the reversing unit so the the whistle turns on only when the controller button is pressed.  Then your customer can choose (or not) direct control of the whistle (press button, whistle ON) rather than the alternating ON-OFF behavior if going thru the reversing unit.

Perhaps someone has come up with a modern replacement module for the combo (DC-sensitive detector and reversing unit).  In other words the module's relay would have the alternating/latching ON-OFF behavior triggered by pressing the Whistle (or Bell) button to turn a rolling-stock feature on and off.