Andrew
Disclaimer: Please keep in mind that although I work in electrical engineering, I have never used DCC myself. I have read many of the forums to try to get a feel for it, but being restricted to small HO and HOn3 steam locomotives, very small layouts, and single operator operations, I haven't taken the plunge yet.
After wading through the Soundtraxx web site and finding precious little data on the DSX, studying the various diagrams, here is what I have concluded. Some of the diagrams show 4 wires connecting the DSX, the Tech Bulletin you cited shows 5. In any case, 4 are identified - 2 for input power and signal, 2 to power the speaker. The 5th, if it exists, would logically be a synch signal from the motor decoder. However, I haven't seen any such output on a motor decoder.
Again, going back to the capacitor installation tech bulletin, note that the capacitor is NOT installed across the power leads. As in my earlier posts, a capacitor of that size directly across the track would likely result in DCC signal degradation. And using an electrolytic capacitor - again cited in the diagram and parts list - means that polarity reversal cannot be allowed. This cannot be guaranteed, even in the best DCC wiring. For other capacitors (speaker and motor), Sountraxx requires use of bipolar capacitors to guard against DC polarity reversal. Therefore, the use of an electrolytic capacitor in this specific case further supports my conclusion that the Tech Bulletin procedure is NOT installing the capacitor across the power leads. The Tech Bulletin mandates the capacitor be installed on 2 external pads with polarity markings, and that if these pads/markings are not present, the modification must be done at the factory.
Based on all the above, I assume there is diode polarity protection inside the decoder(and quite likely other circuits) between the power leads and the circuit insertion point of the capacitor. These internal ciruits would limit the capacitance seen by the track, yet allow the large capacitor to function as a temporary battery, similar to the circuits in VCRs and digital alarm clocks that keep the time while the device is momentarily unplugged or drops power.
Bottom line: You want to avoid excessive capacitance across your track. The signal is quite robust, and at a reasonable frequency, so that the little capacitance of the parallel rails and bus wiring can be shrugged off. But I don't think (my engineering guess) your DCC system will function reliably with 1 or more 220ufd capacitors across the rails.
An analog engineer occasionally lost in a digital world...
Digital is all fantasy anyway - the real world is analog!