Some years ago I posted a short article (here) that mentioned my second American OO locomotive was a modified AHM Plymouth MDT Diesel. It is a simple conversion as all you have to do is pull the wheels out to gauge on the existing axle and create a higher coupler mounting. I ran and ran this model for some years, often in tandem with my kitbashed boxcab diesel, but eventually it did not run that well (I ran it pretty hard) and I had other options.

But when doing some recent testing of how smoothly engines ran around the layout I got it out again. And backing up another step, I had at several recent shows purchased three more similar locomotives, one almost exactly the same, for possible kitbashing into something bigger, but I’ve changed the direction of that project. But then now I had an extra of the AHM model on hand with a fresher drive (but other issues), which is part of the story today.

The AHM Plymouth MDT Diesel has been in production in various forms since 1965, decorated many ways (more information here). I purchased mine at a mall no later than early high school, specifically to use in OO as my eye said it was certainly too big for HO and should be an easy conversion. More thoughts on the actual scale of the model at the end of this article.

Model Power also produced a Plymouth DDT Diesel. It represents an older/smaller prototype model and runs on an updated version of the same drive. And it is absolutely HUGE in HO! Looking online, I see that people often use it unmodified as an On30 model, and I think it is a good move, as I suspect it is oversize for S scale. In OO it is much too big, the prototypes in real life are tiny locomotives. Anyway, I converted the drive of one of these to OO and painted it up nicely, but was never happy, it just did not look right with OO models. I sat it on a shelf. I’m not sure if I ever posted about it to the original blog, as it was a disappointment.

The significant thing I did though was get it out and test it again. The Model Power DDT model has a can motor and really runs quite well, especially considering the price point. And a quick check confirmed that the AHM MDT body would fit the Model Power DDT frame (at least the one I have) easily.

So I got to reworking that model and another HO MDT. I pulled the motor from my original MDT (I could see cracks at the top of it, from overheating) and replaced it with a fresher one, and I put another MDT body (ATSF, for now) with it. The original MDT body, lettered for the Fall River Railway (one of my freelanced lines) is now on the DDT frame with the can motor, seen above. The outcome is I now have two operational MDT models and a box of Plymouth MDT and DDT parts, potentially handy for some future kitbashing project.

It is exciting to see this old friend running so nicely now. It runs slower than the stock model with the new drive, and will creep around the layout, not slowing at certain areas as most of my locomotives do. I’ll post about that at some future time. For sure models with can motors run better than most others for me (in terms of creeping around at a constant speed), and the Model Power drive will improve the AHM MDT model, which looks quite nice with American OO models.

But what scale is it?

I have yet to find a scale drawing of this exact model of Plymouth, but dimensions I see online suggest that the cab of these in general should top out at about 10 over the rails, and the AHM model is around 11’ 6” in OO. The door into the cab is about 6’6” in OO which is reasonable (no bumping my head!). I strongly suspect the model is scaled a bit over OO scale in overall size. The model still looks nice and operates well with large and free rolling American OO models that are not too heavy. Even with only one powered axle it will pull three cars of this type with ease. It is a model for every OO enthusiast to consider.