These are very nice looking models, and a bit of a departure for Overland (not really fitting in with their normal "big, long diesel" oeuvre). As far as I know, these were only sold in A-B and A-B-A sets.
The mechanism, though quite small, is similar in design to the other (larger) Overland/Ajin diesel models of their era. The chassis is metal and very light (with much of the locomotive's heft being provided by the shell). The motor is an open-sided, skew-wound 3-poler. All wheels are geared and provide pickup (IE, no traction tires). Current is transferred from the trucks to a PC board riding above the chassis via long metal contact strips. The PC board then routes current to the motor contacts as well as contacts for the lighting (a directional headlight is mounted inside the shell of the "A" units). Apart from the worms, all gearing is plastic. Flywheels are mounted on both motor shafts. The wheels are low-profile, so no problems on Code-55 track. Micro-Trains couplers are mounted on the pilot-end of the "A" units as well as the back end of the "B" units. A metal drawbar is provided on the back end of the "A" units for permanant coupling with the "B" units.
Paint and detailing are quite exquisite. All windows have glazing, all the handgrabs are separately applied, etc.
Performance is decent (if unspectacular). Pickup is good and throttle response is smooth. On the down side, slow speed creep is fairly mediocre and the top-end speed is excessive. More annoyingly, these things are awfully noisy - like "Bachmann trainset loco" noisy. The flywheels appear to have minimal affect, as these locos tend to start abruptly and stop on a dime.
Overall these are really nice looking models that run "OK". But for my money, I'd certainly expect more. Frankly, I think Intermountain's FT's put these things to shame (at least in the performance department).
To remove the shell, unscrew the four screws (one on each corner) that hold the shell to the chassis. It should lift right off at that point.