My MTH set has a box labeled "front segment", two labeled "center segment", and one "aft segment". That's what the Shuttle SRB motor segments (the ones filled with solid propellant) were called. The SLS SRBs have an added center segment (imaginatively called "center center") so there are ten motor segments in a full SLS stack or train. But I guess the train goes with what ever the manifest says.
The buffer cars are placed between booster cars with LOADED segments for multiple reasons like minimizing risk of damaging multiple segments in a derailment or reducing loading on bridges, but empty cars or cars with expended/cleaned segments (basically big but relatively lightweight tubes) don't require the buffers interspersed. So any train heading back to Utah could be put together in any order that's easiest to switch.
Also, note that in the train with twelve booster cars there are two that are different - the red ones with twelve axles rather than eight. One of the videos mentions that those were test cars for carrying booster segments for a different rocket.
