You are going to have to make some choices on this unit. The SoundTraxx Tsunami is one of the best ways to go.
First off the SD-24 has a prime mover ( diesel engine) that is a EMD 567 with a turbo. I have looked on the SoundTraxx web page and see NO 567 turbo decoders made.
I think that I might go with the EMD 645 which is turbocharged. I would use the TSU-AT1000 decoder as it will be a much easier install. If you use two speakers they should be wired in series.
You might just go with a single speaker and leave one of the weights in. Make sure that the place where you mount the speaker is closed off and will make a speaker enclosure.
Here's a link to the soundtraxx site that helps select the right decoder.
http://www.soundtraxx.com/choose/choose3.php
Springfield PA
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You would get 2 speakers that would be the correct size for the units and wire them to the sound decoder.
QSI and Soundtraxx should both have a decoder that would fit. The speakers would be wired in parallel or series depending on the electrical properties (Ohms)
I have an Atlas SD 24 that is decoder ready ( 8 pin) and a place to mount two speakers. If I want to add DCC control and sound, does anyone offer a plug in package with dual speakers pre wired so it can be dropped in the unit?