In order to unlock this achievement you must complete all of the tutorials in the "Great Western Express" route. So make sure you select that one from the main menu (it's the icon with the green and yellow train on it), then scroll down to "Training."
Then complete the following:
- Train Sim World Introduction
- Great Western Express Introduction
- Class 43 Introduction
- Class 166 Introduction
- Class 66 Introduction
- Station Stopping
There is no medal requirement, I got bronze on two of the three that award them. Pretty straight forward 100GS.
Note: While I was trying to unlock this I somehow wandered over to the expansions included in the Game Pass version, which have their own achievements, but don't count towards this one. After completing all of them I was confused on why this hadn't unlocked. I missed the Class 66 tutorial some how along the way. So just make sure you're doing the tutorials in Great Western Express and you have check marks/medals on all of them.