Iron does not represent actual iron ore. You're not making tools or cars or steel. "Iron" would better be called "tools". Tools/machinery is smaller and pretty much represents a one-time delivery that could be done over regular old roads ... no special mass transport (canals or railroads) required. It doesn't "fly", it drives. Or maybe it's carted.
That's how I explain it thematically, too. But we still all say that iron "teleports". :)