The Whistle/Horn button on a Lionel “Multi-Control” transformers is a two-stage switch. The purpose of the two stages is to first generate a large DC component in the track voltage to pull in the Whistle/Horn relay in the locomotive or tender, then drop the DC voltage to a smaller amount that will hold the relay in the activated state.
When the Whistle/Horn button is activated to stage one, a diode is inserted in series with the output voltage AND an extra “Boost” transformer winding of about 5 volts AC is also inserted in series. The overall output drops slightly because of the loss of half the waveform, but the large DC component pulls in the relay.
The switch is then quickly moved to the final stage to bypass some of the missing AC waveform around the diode. The bypass is a resistance wire that takes different forms in the various Lionel transformers, depending upon the transformer’s peak current rating. This bypass, combined with the Boost voltage, actually raises the output voltage above the original non-whistle voltage. This extra voltage boost was deemed necessary to provide extra power to drive both the locomotive and the whistle motor so that the train does not slow down. Note that the waveform is shifted upward slightly by the remaining DC voltage.
The above waveforms will vary for different models of transformer and the amount of current being sourced. These photos were taken with a Lionel Model RW 110 watt transformer feeding a 6.7 ohm resistive load.
The DC-sensitive relay is an interesting bit of engineering. Note the two heavy copper rings around the core of the relay. These shorting rings give the relay a very high AC impedance because of the large opposing magnetic field generated in the copper rings
by any AC magnetic field. The rings, however, do not impede any DC currents that want to flow since the DC currents do not produce any rapidly changing magnetic fields. The DC magnetic flux operates the relay by pulling in the armature (which is not attached in this photo.) This relay will ignore 20 volts AC, but respond to about 1.5 volts DC!