My Shay truck wires were all red insulation that had been painted black.  The wire was so cheap and stiff it would break with any movement, so I replaced it with some 24 AWG microphone wire which is nice and flexible. The Shay is still running fine after 5 or so years.

I have seen similar wiring on all my Bachmann locos.  The factory seems to use whatever is laying around for wiring.

Cheers,
John