Maretonia is directly land adjacent to the southern border of the Crystal Protectorate. It doesn't matter that trains don't go over water; our current opponent shares a continent with us.
I'm not opposed to air research, but we have limited actions and are about to be engaged in conflict with a military peer. In a choice between trains and airships, trains simply offer a better return on investment in the short term.
I'd also say that they're better in the medium and long term as well, what with mass movement of freight being the number one most important thing for an industrial power, but the short term is what needs to be the focus until we have a better measure of Maretonia. I mean, we don't currently know if we can destroy them or if they're the kind of enemy that will survive a war with us. If they're longer term foes then the short term stops being important, but I digress.
Tl;dr: Choo Choo superior, airship inferior.