Originally Posted by
mnbp
Skymiles are redeemable on a partner's metal, but not on a partner of a partner's metal. The train is a partner of AF/KL.
Not always necessarily so. For example, you can book JL operated flights under the AF codeshare using SkyMiles (or at least you could in the past).
"Partner of partner" is bookable, IF the partner releases award availability for it to Delta (and Delta doesn't block it). That is all that matters.