Thor-I didn't see your post until after I posted mine.
I actually tried exactly that, and generally had continuity, but even by touching the probe to a wheel I may move it enough to get continuity back.
I have a scale lighted caboose with a similar truck/pickup roller spacing that goes through with only a slight flickering (lights never go out). Hence it seems to be an extremely momentary loss of either hot or ground (the flickering), which is exacerbated by the undersized supercap and causes the electronics to shut down...
That said my degree is in civil engineering, not electronics...
