Remember AC output from the old transformers was something like 20 volts alternating polarity, simplistically a sinusoid alternating plus-20, then minus-20 about a nought line.
DC has a polarity rather than an alternating form, so an offset from the nought. Adding a DC component atop the AC form sums the total voltage but only on one side of that nought line.
Where the motor itself won't discriminate the two, a relay as an electromagnet can be pulled by that disparity, so adding that DC "offset" will pull a relay, closing its contacts, and allowing voltage to flow to that motor that powers the whistle mechanism.
A long time since I thought through this so I might have it wrong, but it makes sense working through it again now so I think I still have it right, albeit simplistic.
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