Although there is some voltage drop across the output transistors in the ZW, with no load that drop is quite small.
BUT the Lionel phase control power controllers such as the ZW and the Powermaster reserve a small portion of the AC sinewave for generating the DC offsets for the horn/whistle and bell. The waveform pictures at the end of
http://www.trainfacts.com/trainfacts/?p=180 show how these offsets are created.
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