I agree with all the advice that has been offered about baffles. One additional thing to bear in mind is that the sound set for each locomotive was tuned for the bass response of the system as-shipped. So for instance, if you put a sound board from an NW2, which had a small pancake-style speaker into a super-bass B unit enclosure...well, I'm sure it will be louder and fuller, but it won't go as deep in frequency as you might hope, because those lowest frequencies were intentionally trimmed down in the NW2 sound set.
This isn't meant in any way to discourage experimenting! Just pointing out that the original sound set itself is another factor to keep in mind.
