Key's PRR Q-2's came in two different flavors - the original 1942 version (#6131) and the later 1949 version (#6175). These models are extremely rare and highly prized by Pennsy modelers. And as such, they've been known to sell for well in excess of $1000 on eBay.
The locomotive chassis is all metal and fairly minimalistic (with most of the model's weight coming from its shell). The motor is a coreless Faulhaber. A multi-piece plastic driveshaft runs from the motor to the rear worm housing. A metal driveshaft connects the rear worm housing to the forward one. Both housings are mounted on leaf springs. One axle per engine is geared, with all gearing being metal. All of the driving axles are mounted in bearing blocks. Like the prototype, this model has a non-articulated (rigid) frame and the forward drivers on the rear engine are blind (no flanges). These models can handle curves as narrow as 15"-radius (although any sharper than that and the drivers tend to bind).
Right-rail pickup comes from the five right-side drivers. Left-rail pickup comes from the left-side tender wheels (with current then routed to the locomotive by way of a pair of stiff wires on the drawbar). The pilot and trailing trucks on the locomotive are electrically neutral. As delivered, there are no traction tires (although a TT-equipped driverset is included in the box should you wish to trade pickup for pulling power). The directional headlight is controlled by a PC board mounted inside the boiler shell. Two wires (equipped with plug/socket connectors) run from said PCB to the motor. Wheels are low-profile (no problems on Code-55 rails). As delivered, there is no coupler on the tender. However, a screw is provided for installing a Micro-Trains (or whatever) coupler box. The pilot comes equipped with a tiny (non-operating) knuckle coupler.
These are really gorgeous models that are at least capable of running extremely well. When drawbar current is constant, these locomotives will glide around the rails just as smoothly and quietly as can be. Pulling power is strong, slow-speed creep is outstanding, and I had no problems keeping all of the wheels on the rails. OK, the top-end speed is excessive, but y'know, don't turn the throttle up that high if it's going to bother you.
On the downside, the drawbar (and it's current-conducting wires) can be a problem. As is generally the case with that current transfer scheme, the locomotive will occasionally exhibit some balkiness (or even stall) if the drawbar wires are not properly adjusted and in firm contact with the tender peg. The presumed solution to said problem would be to run an insulated mini-wire from the tender to the motor - providing bulletproof conductivity and making for one sweet running steamer.
To remove the boiler shell, first unscrew the two screws on the back end of the cab. Next, remove the pilot truck and then the (now revealed) screw underneath said truck. The shell should lift off readily at that point.
Grade: B (due to the drawbar issues)
(Photos by John Sing, thanks!)