Informed speculation:
The two cars with HVAC units are probably the two forward segments, one per booster, which have the igniters and some associated avionics installed. I guess that somebody probably decided that it would be better to keep the SLS version nice and comfy.
The Lionel booster set (and the MTH version I have) model the Space Shuttle SRBs and transport. That had four motor segments for one booster packed with solid propellant (forward, forward center, aft center, and aft) plus a car with the nose and aft skirt. I don't believe they usually sent the nose/skirt back and forth to Utah, so a typical Shuttle train had eight cars with just the motor segments. And the forward segment cars did NOT have HVAC units.
The Space Launch System (SLS) version of the SRB has an added fifth motor segment ("center center") so an SLS booster train will typically have ten booster cars (with two having extra amenities). And a "prototypically correct" SLS booster train would have a couple more center segments under the covers.
John
