Introduced: 1972 (revised 1987, 2004 and 2015)
As noted above, there have been four different versions of the Bachmann MDT (that I know of). All versions share the same shell.
The first version came out in the "blue/clear plastic clamshell box" era and is stamped "Bachmann Hong Kong" on the bottom of the chassis -
The chassis is a very light mix of metal and plastic (with an additional free-floating weight sitting atop the worm gear). The truck sideframes are plastic (and actually look quite a bit better than the metal ones found on the later versions). A non-directional headlight is mounted to the front of the chassis (and sadly, pretty much lights up the entire nose from within). All wheels are geared, although only the outer four wheels provide pickup. There are no traction tires. The Rapido-style couplers are mounted to the chassis. Apart from the worm gear (which is brass), all of the rest of the gearing is plastic. The motor is an open-sided 3-poler. The wheel flanges are not overly huge, so these should be able to handle Code-55 track without difficulty.
These first release MDT's are just so-so runners (given their limited pickup footprint and light weight). IE, they'll go around a basic circle of track pretty well, but beyond that they're pretty useless.
The second version was released circa 1987 (during the Bachmann "white cardboard box" era) and is also stamped "Bachmann Hong Kong" on the bottom -
This release has a metal split-frame chassis with an inner plastic frame (providing support for the drive and the couplers). A non-directional headlight is mounted in the front of the chassis. Couplers are body-mounted Rapidos. The truck detail is actually stamped onto the sides of the metal chassis (and as mentioned above, is a bit more primitive looking than the plastic sideframed first release). Like the first version, all six wheels are geared, and four out of the six provide pickup (although, oddly enough, the rear four wheels this time). Performance is, once again, fairly iffy.
Yet another version was released in 2004. The most noteworthy change here is to the pickup - now derived from all six wheels. Additionally, the wheels are "blackened" (which seems like kind of a waste of time since you can't really see them). These models will be found in Bachmann's more modern jewell cases. The bottoms are stamped "Bachmann China". Another spotting difference is the axle gears (black on this version, white on the previous version).
These models were revised yet again circa 2015 (with the main change being a new closed-sided can motor). The chassis was also altered slightly to accommodate said new motor and the couplers were changed over to E-Z Mates -
Performance on this newest version is surprisingly impressive (or at least it is once you run the blackening crap off the wheels). It runs smoothly and quietly at all speeds, and slow-speed creep is quite excellent (although the top-end speed is pretty much off the charts). And despite the small footprint, pickup is damned near perfect (even through turnouts with non-powered frogs). No doubt about it, this is clearly the best of the MDT/WDT-style switchers in N scale (at least as of 2015). I guess the extra pickup was all it really needed.
Performance aside, I don't find any of these models to be particularly convincing in the looks department. So, at best I can only give the post-2004 versions an overall "B" rating (although they definitely run at an "A" level).
Trivia - Although Bachmann has always referred to this model as an "MDT", it is probably more correct to call it a "WDT" (since Plymouth MDT's generally only had two axles, whereas WDT's had three).
More Trivia - These same models were also distributed by Tomy in the 1970s (probably only in Japan). Same basic packaging, but with the Tomy name added. EG -
Shell Removal -
To remove the shell, simply wiggle it up and off (friction being the only thing holding it in place).
Grades: C (for the first two versions) and B (for the rest)