I never said it's simple. I never even said it's necessarily possible, just that it should be considered first, otherwise his solution is like killing mosquitos by means of artillery.
He might have the basic concept of how to go about it down, but it's far from being so straightforward as you claim. It's still a several hundred kilometer route, and it's not like you can just lay down railroads wherever. Even assuming a route that avoids natural barriers etc. is already ploted you still need to do a lot od land engineering to make it suitable, things like hardening the soil, at the very least. Also, he can't really outsource without first tutoring a lot of people how to make devices which they have no concept of. As I've written elsewhere, this is a megaproject for a feudal civilization and it isn't going to be fast no matter what.
The greater problem is that once he shows himself capable of running such a construction, he won't be allowed to do anything else but that, until the country is satisfied. It won't end with one route and that's just the immediete consequence, before the real issues with a medieval kingdom having mass transit rear their heads.
