Considering your somewhat unusual situation of having the knowledge/ability to add custom programmable electronics to your engine, here's another idea. Why not find out where the engine's stock electronics senses the DC offset for bell/whistle. It probably uses an A/D converter or a DC offset comparator to measure a scaled-down version of the track voltage. Then just inject what probably amounts to a milliamp or so of -DC offset into that node when you want to trip the bell. The motor drive DC voltage would be derived from unaltered track voltage so the speed would not change...and there's no diode drops wasting power (not that anyone said wasting power is an issue).
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