I replaced an MRC diesel sound decoder with a Soundtraxx LC in one of my F units and, rather than trash it, I fitted it into a Bachmann railmotor - I believe these were re-engined with EMD prime movers at some stage in their lives.
Here's some pics as to how I did it.
MRC diesel decoder fitted to top of motor
Holes drilled under loco for speaker, mounted under the seating casting, which is used as the lid of the speaker baffle. the baffle sides are made from strip plastic.
A close up of the decoder and the LED which has been attached to the lamp with heat-shrink tubing.
The sound is pretty good using a 3/4" round speaker, and the difficulties associated with synchronising the decoder are not as severe because of the small diameter wheels.
Jon
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Preiser passengers have been painted and fitted - although they sit rather high behind the windows.
The body and roof are refitted and the decoder and speaker are invisible
I'll do some weathering before taking some video of it in action.
Time taken - one afternoon (including painting the passengers.