Not even a 333? Other die cast? What's next? Vanilla ice cream? (Just kidding)
I'm curious why? Are you brand or diesel exclusive? (I get most diesels &tin) Are you a "Shoeless Joe"? Bad experience? I haven't had an AC powered Marx model die on me yet. I cannot say that about ANY other mfg. Lionel PW falls behind 12:1 (?) if a Marx dies here tomorrow.
I started off in O gauge with Marx trains. I didn't like the way they felt, the way they ran, the way they looked... it is a very budget brand and it shows in my opinion.
I agree with you that they are almost impossible to kill, I found one in the middle of the desert, took it home and put some oil on it and it ran! But you probably won't get much of an improvement from a new locomotive from them, with their grindy motors and flimsy construction I'll pass on anything Marx. I do prefer Lionel, but I have some Flyer, Ives, Dorfan, and some Marklin as well.
I've never seen the appeal of them, but I'm sure there are plenty of scale modelers that'll say the same thing about my tinplate collection though, so to each his own.