I suspect my little emitter follower may be more useful than I imagined. Now that I see how easy it it to whack one together, that will no longer be an issue. Of course, for the dummy locomotive board would have the issue of finding a source of 5V to power the circuit.
It's clearly the sound board that is stepping on the signal, it drops the amplitude of the serial data from a bit over 4V p-p to around 3v p-p.