I see it lets us actually build maps from scratch which is neat but is there a way to make custom routes on existing lines like the scenario planner in tsw2? I only have about an hour into this game so far.
You are talking about two different things The Scenario Planner is TSW2 lets you create a Scenario for what you have bought - AFAIK you cannot make a Route in TSW2 In TS22 you can create an entire route or a scenario on any route.
..and custom freeroams, custom quickdrives, custom assets... everything you want. You can even make a timetable scenario in Freeroam which lets you drive which train you want to. Some of Not.Silent's workshop scenarios demonstrate this nicely, drive a train from A to B then swap to another as it enters the station and continue to C.
So I go to make a scenario but for some reason I can't select any routes for the train to take. It always is just grey and won't let me pick one.
You can try tutorials on youtube , there are some basic ones for beginners. www.youtube.com/watch?v=-MlDHGyinKs
You create a scenario by creating a Timetable In the Timetable Editor you tell it which route you wish to take and which places you wish to stop at. If you make a Free Roam Scenario you can go where you want and change the points as you go