While I know some people question the source https://en.wikipedia.org/wiki/FM_H-24-66 shows only 107 units build. The entry also references various mechanical issues. Perhaps Dud is a strong word to use. But for a train that was somewhat obscure I feel like the Lionel model had made this engine much more popular than it was in real life. I think the answer of, IT was big and different may be as good as I am going to get but I think I can live with that. Thank you much!
Well, the Train Master was really was no less obscure than Lionel's non-scale proportioned E33 Rectifier (12 units,) only purchased by the Virginian, or the EP5 (10 units,) only purchased by the New Haven. Had Lionel done an SD7 or 9 instead, most folks back then wouldn't see it as much different than Lionel's GP9.
The railfan community back then wasn't as large or sophisticated as it is today and was still generally more interested in steam than diesel or electrics. I'd wager most folks got their first exposure to the TM, E33 and EP5 locomotives through the Lionel catalogs.
I'll venture a guess that Lionel chose the F-M TrainMaster because (1) it was pretty much the most powerful single-unit road switcher available to the railroads in 1954; (2) It was long, so it made an impressive show, and could command a premium price; (3) most importantly, unlike geeps, Alco RS-3, etc., the long hood was wide enough to acommodate a second Pullmor motor! Dual motors was a feature folks had come to associate with Lionel's premium diesels. It wasn't until the can motor era that we got dual-motored Rectifiers and GP9s.
The EP-5 made its real-life debut a year later, in 1955. It was also impressively long, although it lacked the heft of the FM's die-cast fuel tank. Lionel imaginatively lettered it for Great Northern and the Milwaukee Road, broadening its appeal. Why the EP-5 never got a second motor, I don't know. The shell is certainly wide enough; it would seem like a no-brainer to use motors, power trucks, and pilots from the 2383.
Someday I'll probably add an FM to my collection. I happen to like the newer tooling with the blind middle axle and added detail. Even on sharp curves, I think it looks better than the postwar design, with its inside axle hanging out in the breeze. My $.02.