You're obviously right about the definition of a train. Now that I think about it, the "class lights to markers" definition was a rules class cliche (since the engine needed to have class lights). And while we used the term class lights, the SP rule book actually referred only to "signals" or "green or white lights", or at least that was all I could find. There were several pages of illustrations showing the various combination of flags, lights, and markers for such circumstances as "engine running backward by night, without cars or at the rear of a train pushing cars, and displaying signals for a following section". Lit train number indicators, green flags and green (class) lights, and markers on the pilot bean. Kinda like a Christmas tree. Neat stuff.
Getting back the ng., during its final years the extra trains between Alamosa and Durango/Farmington did not use class lights or flags. Presumably a timetable bulletin or something similar authorized their non-use.