Doesn't appear to be an "official" offering. Although it's possible some dealers and LHS's may offer break-ups. But I don't think you're gonna see dealers ADVERTISE that in print. It's something they may do as a one-off situation if sales are otherwise slow.
I'd say the odds of this happening were better if Lionel didn't offer the #612 J as a stand-alone locomotive offering. That's probably gonna be the easiest way for folks who are only interested in the locomotive to get one. But that doesn't help folks who are only interested in passenger cars. Nor does it make it likely that dealers will even offer break-ups of the set with the #611 J and passenger cars.
Now some folks will advise that you simply purchase the entire set, then try to sell the #611 J. But I'd be careful with that strategy, since (A) we don't know how MANY of those sets are gonna be produced, (B) we don't know how MANY of the #612 J's will be produced, and (C) as we've discussed ad-nauseum here already, these locomotives don't have the enhanced steam effects to which we've grown accustomed to lately. So the locomotives alone may not be in high demand because of that. And you might take a beating on offloading the J in the set.
Just some thoughts to think about...
David