By the looks of the imagery used, Myabi doesn't just have trains in mind, but a full on subway system. A lot of this becomes a lot easier with that in mind. We already know Myabi can just tunnel easily. And tunnels innately will be far more secure than above ground, so monster will be less likely attack the tracks. Also no need to take into account map topography or land ownership. Straight route underground. Four train tracks, two eastward/ westward bound tracks split into commuter and cargo lines, with 1-2 trains per line.
Sure, they'll need some start up capital and governmental approval, but he literally knows the crown Prince. Infrastructure projects are huge economic boon for a country as well. Job creation, investment, ticket revenue, tax on goods. And any under ground caves full of monsters that appear the along the way, will just turn into Adventurers guild quests for Myabi and his party to handle. The hardest part will be getting engineers for the train construction and training drivers. As far we know, Myabi hasn't shown the ability to create something as complex as train. The parts for one sure, but functioning train is harder... Then again, magic trains designed by the professor(s) from the academy?