With the Mastery, you can unlock dioramas. I'm not sure what they do. Are the like the Depot and Rural backdrops - do your trains appear there when you select them?
If ya can get mastery working, yes. What you've described is exactly what they are. Some of them look pretty nifty being honest but I've never managed to get mastery to track...
The level 2 mastery unlocks special things, for example on NTP you get some hot air balloons near Dewsbury or GWE the festival near Reading. This is all, as Matzo says providing it works!