OK to run wire beneath / along track lighting?
To start I would like to say that I don't believe you should do any expansion to Knob and Tube unless you plan to at least upgrade that circuit to newer methods.

With that out of the way you can run circuits though lighting fixture and track, but only if it meets certain qualifications and that will be approved by the manufacturer.

Second I don't know of any wiring method that would allow you to run wire underneath a track.

If you really don't care much about how it looks I would suggest an exterior wiring method like "wiremold".