Between BB and RTR (Ready to Roll) there was a transition model. It used the BB chassis, motor and metal strip connection method of those older types. The shell still had the gap in the pilot beneath the coupler and was held to the chassis by 4 clips that extended down through the fuel tank, motor recess in the chassis. I only had 1 of this type and swore I'd never get another, because of the great difficulty disengaging those 4 clips without them popping back on whilst doing it. They did have wire grabs added to the shell, instead of the BB's moulded on ones, and a choice of the BB wire hand rails, or the new, finer detailed, Delrin plastic handrails/stanchions. Having the BB motor/lighting system, they are not so easily converted to DCC. They came in an upgraded box with a viewing window. They are visually identifiable by that gap in the pilots below the couplers.
The next models up are the ones usually referred to as RTR/DCC Ready. These used the same box as that just described but with the addition of a large yellow, multi pointed, Star in the upper right corner of the window. On the star is written that the model is DC/DCC Ready. DCC Quick Plug Equipped. These, if you are looking to install DCC, are the ones to look for. The old metal strip connectors are gone, replaced by a circuit board and wiring, directional lighting and a choice of either an 8 pin NMRA socket, or a 9 pin socket on a wiring tail to plug a suitable decoder into. New shells were also introduced with better, upgraded tooling (the nose hoods were slimmer and close to prototype scaling), all wire grabs installed, including metal cab sunshades and only the Delrin handrails. The gap beneath the couplers was closed and the shell held in place by the screws through the coupler boxes. The chassis had more weight and the wheels were now NS. The motor was still open, but the armature was upgraded. You will still find them for sale, some still New in the Box. Oh, and opening, sliding windows in the cab sides.
The Genesis Line came into being around the same, using the identical shells, but a better Can motor (about 2008). They also came either DCC ready or fitted with MRC DCC/Sound ex factory. Those decoders were not regarded highly for reliability. In about 2011, the sound decoders were changed to the first edition Soundtraxx Tsunami. Identifiers between the 2 versions, is mainly the boxes they came in. The Dcc ready or MRC fitted had a yellow and blue box (emphasis on the yellow). The Tsunami fitted was a Solid, one color, dark blue box. That they have sound decoders is noted on the end of the boxes in both cases.