"There are apparently two types of 3-rail Train Masters. Lots of differences, but let me offer a major spotting feature: the original Lionel, and Williams and MTH reproduction, all have little square openings in the aft hood where the fans are.
K-Line went whole hog and did a very good scale version, and back where those square holes were, K-Line put screens - very high quality realistic screens. k-Line also had six axle drive - critical for us 2-railers."
There are/were 2 sets of MTH Train Master tooling:
1 - the model of a model version, which emulates - like the Williams - the PW Lionel version, big screw head and all, as I understand. I do not have one.
2 - the model of an FM Train Master version, which models, well, the actual locomotive (as does the K-line version). I do have one of these, a Southern Railway dummy. Furthermore, MTH is the only one to have properly modeled the SOU version, as that railroad had a different pilot deck arrangement from most (all?) other FM TM's. I was impressed that they did this. The K-line SOU version looks very nice, but has incorrect pilot features for a SOU Train Master.
I found the SOU MTH TM hard to find, especially at a rational price, a couple of years ago, when my dummy version came up at a high but not painful price, so I grabbed it.
The good MTH and the K-line versions are much nicer models than the PW emulations, so buyers need to be careful, if it matters. I almost bid on the wrong type when I was shopping for my SOU version. The PW versions leave me nonplussed.