You know, one of my biggest pet peeves in action-adventures is when the good guys suddenly have a hard time fighting foes that are blatantly below their level (and the level of villains they’ve fought previously) because the plot needs conflict by any means. Yes, the goons on this train have guns and there are hostages around. Yes, the implication is that Mustang set these kids up with a little problem to test them. Still, I don’t buy that Ed & Al couldn’t just snap their fingers and be done with these guys.
This sort of complaint can easily sink into a versus thread diatribe, sure, but it more often arises because an adventure story hasn’t got its own rules straight - - like an anime that’s getting made before its own source material is finished, perhaps?
I remember one of the bigger complaints about BROTHERHOOD was that Hughes didn’t get properly established before he was killed off and we were still expected to be pretty upset about it (Mustang certainly was, anyway). He makes a bigger impression with his debut in this show, certainly, and I expect we’ll be made to care just a little more about him before the Homunculi do him in (if that does actually happen in this version?). Past that, this feels like a kind of needlessly-obfuscating way to get us to Mustang, though.
Both iterations of FMA have had a funny conceit of just presenting both these kids as wunderkinds with prestige that’s well beyond their years - - far enough beyond that even stretches disbelief a little in a show about matter-molding alchemists. BROTHERHOOD got around it by just introducing them at their highest rank first and then just eluding to a number of prior adventures they’d gone on.
This one tries it somewhere in the middle and, as a result, it seems real odd that they wouldn’t have already been introduced to Mustang at this point. It feels less like filler and more like filibuster in a show that’s already seeming a little aimless in comparison to sharp mega-arc conspiracy focus that the other version hit right away, I've got to say.